Gas separation using organic-vapor-resistant membranes and PSA

ABSTRACT

A process for separating a gas from a gas mixture containing an organic compound gas or vapor by means of a hybrid separation combining adsorption with membrane gas separation, using membranes selective for the gas over the organic compound. The membranes use a selective layer made from a polymer having repeating units of a fluorinated cyclic structure of an at least 5-member ring, and demonstrate good resistance to plasticization by the organic components in the gas mixture under treatment.

[0001] This application is a continuation-in-part of Ser. No.09/574,420, filed May 19, 2000 and Ser. No. 09/574,303 filed May 19,2000, both of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

[0002] The invention relates to the separation of gases from hydrocarbongas mixtures, such separations including hydrogen from hydrocarbons,carbon dioxide from hydrocarbons, and hydrocarbons from one another. Theseparation is carried out using hydrocarbon-resistant membranes, and isuseful in refineries, petrochemical plants, natural gas fields and thelike.

BACKGROUND OF THE INVENTION

[0003] Polymeric gas-separation membranes are well known and are in usein such areas as production of oxygen-enriched air, production ofnitrogen from air, separation of carbon dioxide from methane, hydrogenrecovery from various gas mixtures and removal of organic vapors fromair or nitrogen.

[0004] The preferred membrane for use in any gas-separation applicationcombines high selectivity with high flux. Thus, the membrane-makingindustry has engaged in an ongoing quest for polymers and membranes withimproved selectivity/flux performance. Many polymeric materials areknown that offer intrinsically attractive properties. That is, when thepermeation performance of a small film of the material is measured underlaboratory conditions, using pure gas samples and operating at modesttemperature and pressure conditions, the film exhibits high permeabilityfor some pure gases and low permeability for others, suggesting usefulseparation capability.

[0005] Unfortunately, gas separation in an industrial plant is seldom sosimple. The gas mixtures to which the separation membranes are exposedmay be hot, contaminated with solid or liquid particles, or at highpressure, may fluctuate in composition or flow rate or, more likely, mayexhibit several of these features. Even in the most straightforwardsituation possible, where the gas stream to be separated is atwo-component mix, uncontaminated by other components, at ambienttemperature and moderate pressure, one component may interact with themembrane in such a way as to change the permeation characteristics ofthe other component, so that the separation factor or selectivitysuggested by the pure gas measurements cannot be achieved. In gasmixtures that contain condensable components, it is frequently, althoughnot always, the case that the mixed gas selectivity is lower, and attimes considerably lower, than the ideal selectivity. The condensablecomponent, which is readily sorbed into the polymer matrix, swells or,in the case of a glassy polymer, plasticizes the membrane, therebyreducing its selective capabilities. A technique for predicting mixedgas performance under real conditions from pure gas measurements withany reliability has not yet been developed.

[0006] A good example of these performance problems is the separationofhydrogen from mixtures containing hydrogen, methane and otherhydrocarbons. Increasing reliance on low-hydrogen, high-sulfur crudes,coupled with tighter environmental regulations, has raised hydrogendemand in refineries. This is primarily due to increasedhydrodesulfurization and hydrocracking; as a result many refineries arenow out of balance with respect to hydrogen supply. At the same time,large quantities of hydrogen-containing off-gas from refinery processesare currently rejected to the refinery's fuel gas systems. Besides beinga potential source of hydrogen, these off-gases contain hydrocarbons ofvalue, for example, as liquefied petroleum gas (LPG) and chemicalfeedstocks.

[0007] The principal technologies available to recover hydrogen fromthese off-gases are cryogenic separation, pressure swing adsorption(PSA), and membrane separation. Membrane gas separation, the newest, isbased on the difference in permeation rates of gas components through aselective membrane. Many membrane materials are much more permeable tohydrogen than to other gases and vapors. One of the first applicationsof gas separation membranes was recovery of hydrogen from ammonia plantpurge streams, which contain hydrogen and nitrogen. This is an idealapplication for membrane technology, because the membrane selectivity ishigh, and the feed gas is clean (free of contaminants, such as heavierhydrocarbons). Another successful application is to adjusthydrogen/carbon monoxide or hydrogen/methane ratios for synthesis gasproduction. Again, the feed gas is free of heavy hydrocarbon compounds.

[0008] Application of membranes to refinery separation operations hasbeen much less successful. Refinery gas streams contain contaminantssuch as water vapor, acid gases, olefins, aromatics, and other organics.At relatively low concentrations, these contaminants cause membraneplasticization and loss of selectivity. At higher concentrations theycan condense on the membrane and cause irreversible damage to it. When afeedstream containing such components and hydrogen is introduced into amembrane system, the hydrogen is removed from the feed gas into thepermeate and the gas remaining on the feed side becomes progressivelyenriched in hydrocarbons, raising the dewpoint. For example, if thetotal hydrocarbon content increases from 60% in the feed gas to 85% inthe residue gas, the dewpoint may increase by as much as 25° C. or more,depending on hydrocarbon mix. Maintaining this hydrocarbon-rich mixtureas gas may require it to be maintained at high temperature, such as 60°C., 70° C., 80° C. or even higher, which is costly and may itselfeventually adversely affect the mechanical integrity of the membrane.Failure to do this means the hydrocarbon stream may enter theliquid-phase region of the phase diagram before it leaves the membranemodule, and condense on the membrane surface, damaging it beyondrecovery.

[0009] Even if the hydrocarbons are kept in the gas phase, separationperformance may fall away completely in the presence of hydrocarbon-richmixtures. These issues are discussed, for example, in J. M. S. Henis,“Commercial and Practical Aspects of Gas Separation Membranes” Chapter10 of D.R. Paul and Y. P. Yampol'skii, Polymeric Gas SeparationMembranes, CRC Press, Boca Raton, 1994. This reference gives upperlimits on various contaminants in streams to be treated by polysulfonemembranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation ofaromatics, 25% saturation of olefins and 11° C. above paraffin dewpoint(pages 473-474).

[0010] A great deal of research has been performed on improved membranematerials for hydrogen separation. A number of these materials appear tohave significantly better properties than the original cellulose acetateor polysulfone membranes. For example, modem polyimide membranes havebeen reported with selectivity for hydrogen over methane of 50 to 200,as in U.S. Pat. Nos. 4,880,442 and 5,141,642. Unfortunately, thesematerials appearto remain susceptible to severe loss of performancethrough plasticization and to catastrophic collapse if contacted byliquid hydrocarbons. Several failures have been reported in refineryapplications where these conditions occur. This low process reliabilityhas caused a number of process operators to discontinue applications ofmembrane separation for hydrogen recovery.

[0011] Another example of an application in which membranes havedifficulty delivering and maintaining adequate performance is theremoval of carbon dioxide from natural gas. Natural gas provides morethan one-fifth of all the primary energy used in the United States, butmuch raw gas is “subquality”, that is, it exceeds the pipelinespecifications in nitrogen, carbon dioxide and/or hydrogen sulfidecontent. In particular, about 10% of gas contains excess carbon dioxide.Membrane technology is attractive for removing this carbon dioxide,because many membrane materials are very permeable to carbon dioxide,and because treatment can be accomplished using the high wellhead gaspressure as the driving force for the separation. However, carbondioxide readily sorbs into and interacts strongly with many polymers,and in the case of gas mixtures such as carbon dioxide/methane withother components, the expectation is that the carbon dioxide at leastwill have a swelling or plasticizing effect, thereby adversely changingthe membrane permeation characteristics. These issues are againdiscussed in the Henis reference cited above.

[0012] In the past, cellulose acetate, which can provide a carbondioxide/methane selectivity of about 10-20 in gas mixtures at pressure,has been the membrane material of choice for this application, and about100 plants using cellulose acetate membranes are believed to have beeninstalled. Nevertheless, cellulose acetate membranes are not withoutproblems. Natural gas often contains substantial amounts of water,either as entrained liquid, or in vapor form, which may lead tocondensation within the membrane modules. However, contact with liquidwater can cause the membrane selectivity to be lost completely, andexposure to water vapor at relative humidities greater than only about20-30% can cause irreversible membrane compaction and loss of flux. Thepresence of hydrogen sulfide in conjunction with water vapor is alsodamaging, as are high levels of C₃₊ hydrocarbons. These issues arediscussed in more detail in U.S. Pat. No. 5,407,466, columns 2-6, whichpatent is incorporated herein by reference.

[0013] Yet another challenging area is the separation of mixtures oflight hydrocarbon vapors. For example, olefins, particularly ethyleneand propylene, are important chemical feedstocks. About 17.5 milliontons of ethylene and 10 million tons of propylene are produced in theUnited States annually, much as a by-product of petrochemicalprocessing. Before they can be used, the raw olefins must be separatedfrom mixtures containing saturated hydrocarbons and other components.Currently, separation of olefin/paraffin mixtures is usually carried outby distillation. The low relative volatilities of the components makethis process costly and complicated; distillation columns are typicallyup to 300 feet tall and the process is very energy-intensive. Moreeconomical separation processes are needed.

[0014] Using a membrane to separate olefins from paraffins is analternative to distillation that has been considered. However, theseparation is difficult because of the similar molecular sizes andcondensabilities of the components, as well as the challenge ofoperating the membranes in a hydrocarbon-rich environment, and nomaterial that can provide adequate performance with real vapor mixturesunder pressure has been found.

[0015] Thus, the need remains for membranes that will provide andmaintain adequate performance under the conditions of exposure toorganic vapors, and particularly C₃₊ hydrocarbons, that are commonplacein refineries, chemical plants, or gas fields.

[0016] Films or membranes made from fluorinated polymers having a ringstructure in the repeat unit are known. For example:

[0017] 1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass,disclose various perfluorinated polymers having repeating units ofperfluorinated cyclic ethers, and cite the gas-permeation properties ofcertain of these, as in column 8, lines 48-60 of 4,910,276.

[0018] 2. A paper entitled “A study on perfluoropolymer purification andits application to membrane formation” (V. Arcella et al., Journal ofMembrane Science, Vol. 163, pages 203-209 (1999)) discusses theproperties ofmembranes made from a copolymer oftetrafluoroethylene and adioxole. Gas permeation data for various gases are cited.

[0019] 3. European Patent Application 0 649 676 A1, to L'Air Liquide,discloses post-treatment of gas separation membranes by applying a layerof fluoropolymer, such as a perfluorinated dioxole, to seal holes orother defects in the membrane surface.

[0020] 4. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separationmethods using perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. Thispatent also discloses comparative data for membranes made fromperfluoro(2-methylene-4-methyl-1,3-dioxolane) polymer (Example XI).

[0021] 5. A paper entitled “Gas and vapor transport properties ofamorphous perfluorinated copolymer membranes based on2,2-bistrifluoromethyl-4,5-difluoro- 1,3-dioxole/tetrafluoroethylene”(I. Pinnau et al., Journal of Membrane Science, Vol. 109, pages 125-133(1996)) discusses the free volume and gas permeation properties offluorinated dioxole/tetrafluoroethylene copolymers compared withsubstituted acetylene polymers. This reference also shows thesusceptibility ofthis dioxole polymer to plasticization by organicvapors and the loss of selectivity as vapor partial pressure in a gasmixture increases (FIGS. 3 and 4).

[0022] Most of the data reported in the prior art references listedabove are for permanent gases, carbon dioxide and methane, and referonly to measurements made with pure gases. The data reported in item 5indicate that even these fluorinated polymers, which are characterizedby their chemical inertness, appear to be similar to conventionalhydrogen-separating membranes in their inability to withstand exposureto propane and heavier hydrocarbons.

SUMMARY OF THE INVENTION

[0023] The invention is a process for separating a gas from a gasmixture containing an organic vapor or vapors. The gas mixture comprisesthe gas that is desired to be separated and other vapor component orcomponents, of which at least one is usually a C₃₊ hydrocarbon asdefined below. The separation is carried out by running a stream of thegas mixture across a membrane that is selective for the desired gas tobe separated over another component or components. The process results,therefore, in a permeate stream enriched in the desired gas and aresidue stream depleted in that gas. The process differs from processespreviously available in the art in that:

[0024] (i) the membranes are able to maintain useful separationproperties in the presence of organic vapors, particularly C₃₊hydrocarbon vapors, even at high levels in the gas mixture, and

[0025] (ii) the membranes can recover from accidental exposure to liquidorganic compounds.

[0026] To provide these attributes, the membranes used in the process ofthe invention are made from a glassy polymer or copolymer. The polymeris characterized by having repeating units of a fluorinated, cyclicstructure, the ring having at least five members. The polymer is furthercharacterized by a fractional free volume no greater than about 0.3 andpreferably by a glass transition temperature, Tg, of at least about 100°C. Preferably, the polymer is perfluorinated.

[0027] In the alternative, the membranes selective for the desired gasare characterized in terms of their selectivitybefore and after exposureto liquid hydrocarbons. Specifically, the membranes have a post-exposureselectivity for the desired gas over the gaseous hydrocarbon from whichit is desired to separate the gas, after exposure of the separationmembrane to a liquid hydrocarbon, for example, toluene, and subsequentdrying, that is at least about 60%, 65% or even 70% of a pre-exposureselectivity for the desired gas over the gaseous hydrocarbon, the pre-and post-exposure selectivities being measured with a test gas mixtureof the same composition and under like conditions.

[0028] In this case, the selective layer is again made from an amorphousglassy polymer or copolymer with a fractional free volume no greaterthan about 0.3 and a glass transition temperature, Tg, of at least about100° C. The polymer is fluorinated, generally heavily fluorinated, bywhich we mean having a fluorine:carbon ratio of atoms in the polymer ofat least about 1:1. Preferably, the polymer is perfluorinated. In thiscase the polymer need not incorporate a cyclic structure.

[0029] In a basic embodiment, the process of the invention includes thefollowing steps:

[0030] (a) bringing a gas mixture comprising a desired gas and anorganic vapor into contact with the feed side of a separation membranehaving a feed side and a permeate side, the membrane having a selectivelayer comprising:

[0031] a polymer comprising repeating units having a fluorinated cyclicstructure of an at least 5-member ring, the polymer having a fractionalfree volume no greater than about 0.3;

[0032] (b) providing a driving force for transmembrane permeation;

[0033] (c) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the gas mixture;

[0034] (d) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the gas mixture.

[0035] In the alternative, a basic embodiment of the process includesthe following steps:

[0036] (a) bringing a gas mixture comprising a desired gas and anorganic vapor into contact with the feed side of a separation membranehaving a feed side and a permeate side, the membrane having a selectivelayer comprising a polymer having:

[0037] (i) a ratio of fluorine to carbon atoms in the polymer greaterthan 1:1;

[0038] (ii) a fractional free volume no greater than about 0.3; and

[0039] (iii) a glass transition temperature of at least about 100° C.;and the separation membrane being characterized by a post-exposureselectivity for the desired gas over the organic vapor, after exposureof the separation membrane to liquid toluene and subsequent drying, thatis at least about 65% of a pre-exposure selectivity for the desired gasover the organic vapor, as measured pre- and post-exposure with a testgas mixture of the same composition and under like conditions;

[0040] (b) providing a driving force for transmembrane permeation;

[0041] (c) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the gas mixture;

[0042] (d) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the gas mixture.

[0043] The permeating desired gas may be either a valuable gas that itis desired to retrieve as an enriched product, or a contaminant that itis desired to remove. Thus either the permeate stream or the residuestream, or both, may be the useful products of the process. Gases thatmay be separated from C₃₊ hydrocarbons by the process include, but arenot limited to, hydrogen, nitrogen, oxygen, air, argon, carbon dioxide,methane, ethane, light olefins and light hydrocarbon isomers. Examplesof C₃₊ hydrocarbon vapors from which the gas may be separated include,but are not limited to, paraffins, both straight and branched, forexample, propane, butanes, pentanes, hexanes; olefins and otheraliphatic unsaturated organics, for example, propylene, butene; aromatichydrocarbons, for example, benzene, toluene, xylenes; vapors ofhalogenated solvents, for example, methylene chloride,perchloroethylene; alcohols; ketones; and diverse other volatile organiccompounds.

[0044] Particularly preferred materials for the selective layer of themembrane used to carry out the process of the invention are amorphoushomopolymers of perfluorinated dioxole, dioxolanes or cyclic alkylethers, or copolymers of these with tetrafluoroethylene. Specific mostpreferred materials are copolymers having the structure:

[0045] where x and y represent the relative proportions of the dioxoleand the tetrafluoroethylene blocks, such thatx+y=1.

[0046] A second highly preferred material has the structure:

[0047] where n is a positive integer.

[0048] Contrary to what would be expected from the data presented in thePinnau et al. Journal of Membrane Science paper, we have unexpectedlyfound that membranes formed from fluorinated cyclic polymers ascharacterized above can withstand exposure to C₃₊ hydrocarbons wellenough to provide useful separation capability for gas mixtures thatinclude C₃₊ hydrocarbon vapors. This resistance persists even when theC₃₊ hydrocarbons are present at high levels, such as 5%, 10%, 15% oreven more.

[0049] A particularly important advantage of the invention is that themembranes can retain selectivity for desired gases, such as hydrogen,nitrogen, carbon dioxide, methane, or light olefin, even in the presenceof streams rich in, or even essentially saturated with, C₃₊ hydrocarbonvapors. This distinguishes these membrane materials from all othermembrane materials previously used commercially for such separations.Membranes made from fluorinated dioxoles have been believed previouslyto behave like conventional membrane materials in suffering fromdebilitating plasticization in a hydrocarbon containing environment, tothe point that they may even become selective for hydrocarbons overpermanent gas even at moderate C₃₊ hydrocarbon partial pressures. Wehave discovered that this is not the case for the membranes taughtherein. This unexpected result is achieved because the membranes used inthe invention are unusually resistant to plasticization by hydrocarbonvapors.

[0050] The membranes are also resistant to contact with liquidhydrocarbons, in that they are able to retain their selectivity forhydrogen over methane after prolonged exposure to liquid toluene, forexample. This is a second beneficial characteristic that differentiatesthe processes of the invention from prior art processes. In the past,exposure of the membranes to liquid hydrocarbons frequently meant thatthe membranes were irreversibly damaged and had to be removed andreplaced.

[0051] These unexpected and unusual attributes render the process of theinvention useful in situations where it was formerly difficult orimpractical for membrane separation to be used, or where membranelifetimes were poor.

[0052] Because the preferred polymers are glassy and rigid, anunsupported film of the polymer may be usable in principle as asingle-layer gas separation membrane. However, such layer will normallybe far too thick to yield acceptable transmembrane flux, and inpractice, the separation membrane usually comprises a very thinselective layer that forms part of a thicker structure, such as anasymmetric membrane or a composite membrane. The making of these typesof membranes is well known in the art. If the membrane is a compositemembrane, the support layer may optionally be made from a fluorinatedpolymer also, making the membrane a totally fluorinated structure andenhancing chemical resistance. The membrane may take any form, such ashollow fiber, which may be potted in cylindrical bundles, or flatsheets, which may be mounted in plate-and-frame modules or formed intospiral-wound modules.

[0053] The driving force for permeation across the membrane is thepressure difference between the feed and permeate sides, which can begenerated in a variety of ways. The pressure difference may be providedby compressing the feedstream, drawing a vacuum on the permeate side, ora combination of both. The membrane is able to tolerate high feedpressures, such as above 200 psia, 300 psia, 400 psia or more. Asmentioned above, the membrane is able to operate satisfactorily in thepresence of C₃₊ hydrocarbons at high levels. Thus the partial pressureof the hydrocarbons in the feed may be close to saturation. For example,depending on the mix of hydrocarbons and the temperature of the gas, theaggregate partial pressure of all C₃₊ hydrocarbons in the gas might beas much as 10 psia, 15 psia, 25 psia, 50 psia, 100 psia, 200 psia ormore. Expressed as a percentage of the saturation vapor pressure at thattemperature, the partial pressure of hydrocarbons, particularly C₃₊hydrocarbons, may be 20%, 30%, 50% or even 70% or more of saturation.

[0054] The membrane separation process may be configured in manypossible ways, and may include a single membrane unit or an array of twoor more units in series or cascade arrangements. The processes of theinvention also include combinations of the membrane separation processdefined above with other separation processes, such as adsorption,absorption, distillation, condensation or other types of membraneseparation.

[0055] In another aspect, the invention is a process for separatinghydrogen from organic vapors in a multicomponent mixture containing atleast hydrogen and one or more organic compounds. Such a mixture mighttypically, but not necessarily, be found as a petrochemical plant or arefinery process or waste stream, such as streams from reformers,crackers, hydroprocessors and the like.

[0056] The process involves running a stream containing hydrogen acrossthe feed side of a membrane that is selectively permeable to thehydrogen over the hydrocarbons in the stream. The hydrogen isconcentrated in the permeate stream; the residue stream is thuscorrespondingly depleted of hydrogen. The process can separate hydrogenfrom methane, hydrogen from C₂₊ hydrocarbon vapors, hydrogen from C₃₊hydrocarbon vapors, or any combination of these.

[0057] The process differs from previous hydrogen/hydrocarbon separationprocesses in the nature of the membrane that is used. The membranes are,as described above, able to maintain useful separation properties in thepresence of organic vapors at high activity, and able to recover fromaccidental exposure to liquid hydrocarbons.

[0058] The scope of the invention in this aspect is not intended to belimited to any particular gas streams, but to encompass any situationwhere a gas stream containing hydrogen and hydrocarbon gas is to beseparated. The composition of the gas may vary widely, from a mixturethat contains minor amounts of hydrogen in admixture with varioushydrocarbon components, including relatively heavy hydrocarbons, such asC₅-C₈ hydrocarbons or heavier, to a mixture of mostly hydrogen, such as80% hydrogen, 90% hydrogen or above, with methane and other very lightcomponents.

[0059] The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbonvapor, for hydrogen over methane of at least about 10, for hydrogen overpropane of at least about 50, and for hydrogen over n-butane of at leastabout 100. Frequently, the hydrogen/methane selectivity achieved is 20or more, even in the presence of significant concentrations of C₃₊hydrocarbons.

[0060] In yet another aspect, the invention is a process for separatingcarbon dioxide from methane and other hydrocarbons. Such a mixture mightbe encountered during the processing of natural gas, of associated gasfrom oil wells, or of certain petrochemical streams, for example.

[0061] The process involves running a stream containing carbon dioxideacross the feed side of a membrane that is selectively permeable to thecarbon dioxide over the methane and the C₃₊ hydrocarbon vapors in thestream. The carbon dioxide is concentrated in the permeate stream; theresidue stream is thus correspondingly depleted of carbon dioxide.

[0062] The process differs from previous carbon dioxide/methaneseparation processes in the nature of the membrane that is used. Themembranes are, as described above, able to maintain useful separationproperties in the presence of C₃₊ hydrocarbon vapor at high partialpressure, and able to recover from accidental exposure to liquidhydrocarbons. The membranes are also able to withstand high partialpressures of carbon dioxide.

[0063] The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbonvapor, for carbon dioxide over methane of at least about 5, even at highcarbon dioxide activity. Frequently, the carbon dioxide/methaneselectivity achieved is 10 or more, and may be as much as 15 or more,even in the presence of significant concentrations of C₃₊ hydrocarbons.

[0064] In yet another aspect, the invention is a process for separatingolefins from paraffins, particularly propylene from propane. Suchmixtures are found as olefin manufacturing effluent streams, and invarious petrochemical plant streams, for example.

[0065] The process involves running a stream comprising propylene andpropane across the feed side of a membrane that is selectively permeableto propylene. The propylene is concentrated in the permeate stream; theresidue stream is thus correspondingly depleted of propylene.

[0066] The process typically provides a propylene/propane selectivityofat least about 2.5, and more preferably at least about 3, which can besustained, even with streams composed entirely of C₃₊ hydrocarbons, overa range of pressures.

[0067] Other separation processes that can be carried out within thescope of the invention include, but are not limited to, separation ofother permanent gases, for example, nitrogen, oxygen, air or argon, fromorganics; separation of methane from C₃₊ organics; and separation ofisomers from one another.

[0068] In another aspect, the invention is a process that combinesmembrane separation with adsorption, particularly pressure swingadsorption (PSA).

[0069] In this aspect, the invention has two basic embodiments. In thefirst, the membrane separation step precedes the adsorption step. Thus,the process includes the following steps:

[0070] (a) bringing a gas mixture comprising a desired gas and anorganic vapor into contact with the feed side of a separation membranehaving a feed side and a permeate side, the membrane having a selectivelayer comprising:

[0071] a polymer comprising repeating units having a fluorinated cyclicstructure of an at least 5-member ring, the polymer having a fractionalfree volume no greater than about 0.3;

[0072] (b) providing a driving force for transmembrane permeation;

[0073] (c) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the gas mixture;

[0074] (d) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the gas mixture;

[0075] (e) passing at least a portion of the permeate stream as a feedstream to an adsorption unit adapted to preferentially sorb the organicvapor;

[0076] (f) withdrawing from the adsorption unit a non-adsorbed productstream enriched in the desired gas compared to the gas mixture.

[0077] In the alternative, the membrane/adsorption hybrid processincludes the following steps:

[0078] (a) bringing a gas mixture comprising a desired gas and anorganic vapor into contact with the feed side of a separation membranehaving a feed side and a permeate side, the membrane having a selectivelayer comprising a polymer having:

[0079] (i) a ratio of fluorine to carbon atoms in the polymer greaterthan 1:1;

[0080] (ii) a fractional free volume no greater than about 0.3; and

[0081] (iii) a glass transition temperature of at least about 100° C.;and the separation membrane being characterized by a post-exposureselectivity for the desired gas over the organic vapor, after exposureof the separation membrane to liquid toluene and subsequent drying, thatis at least about 65% of a pre-exposure selectivity for the desired gasover the organic vapor, as measured pre- and post-exposure with a testgas mixture of the same composition and under like conditions;

[0082] (b) providing a driving force for transmembrane permeation;

[0083] (c) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the gas mixture;

[0084] (d) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the gas mixture;

[0085] (e) passing at least a portion of the permeate stream as a feedstream to an adsorption unit adapted to preferentially sorb the organicvapor;

[0086] (f) withdrawing from the adsorption unit a non-adsorbed productstream enriched in the desired gas compared to the gas mixture.

[0087] In the second hybrid embodiment, the membrane separation stepfollows the adsorption step. Thus, the process includes the followingsteps:

[0088] (a) passing a gas mixture comprising a desired gas and an organicvapor into an adsorption unit adapted to preferentially sorb the organicvapor;

[0089] (b) withdrawing from the adsorption unit a non-adsorbed productstream enriched in the desired gas compared to the gas mixture;

[0090] (c) withdrawing from the adsorption unit a tail gas streamdepleted in the desired gas compared to the gas mixture;

[0091] (d) bringing at least a portion of the tail gas stream intocontact with the feed side of a separation membrane having a feed sideand a permeate side, the membrane having a selective layer comprising:

[0092] a polymer comprising repeating units having a fluorinated cyclicstructure of an at least 5-member ring, the polymer having a fractionalfree volume no greater than about 0.3;

[0093] (e) providing a driving force for transmembrane permeation;

[0094] (f) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the tail gas stream;

[0095] (g) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the tail gas stream.

[0096] In the alternative, the second hybrid embodiment includes thefollowing steps:

[0097] (a) passing a gas mixture comprising a desired gas and an organicvapor into an adsorption unit adapted to preferentially sorb the organicvapor;

[0098] (b) withdrawing from the adsorption unit a non-adsorbed productstream enriched in the desired gas compared to the gas mixture;

[0099] (c) withdrawing from the adsorption unit a tail gas streamdepleted in the desired gas compared to the gas mixture;

[0100] (d) bringing at least a portion of the tail gas stream intocontact with the feed side of a separation membrane having a feed sideand a permeate side, the membrane having a selective layer comprising apolymer having:

[0101] (i) a ratio of fluorine to carbon atoms in the polymer greaterthan 1:1;

[0102] (ii) a fractional free volume no greater than about 0.3; and

[0103] (iii) a glass transition temperature of at least about 1 00° C.;and the separation membrane being characterized by a post-exposureselectivity for the desired gas over the organic vapor, after exposureof the separation membrane to liquid toluene and subsequent drying, thatis at least about 65% of a pre-exposure selectivity for the desired gasover the organic vapor, as measured pre- and post-exposure with a testgas mixture of the same composition and under like conditions;

[0104] (e) providing a driving force for transmembrane permeation;

[0105] (f) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the tail gas stream;

[0106] (g) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the tail gas stream.

[0107] It is an object of the present invention to provide a hybridmembrane separation/adsorption process for separation of gases from gasmixtures containing C₃₊ hydrocarbon vapors. Additional objects andadvantages of the invention will be apparent from the description belowto those of ordinary skill in the art.

[0108] It is to be understood that the above summary and the followingdetailed description are intended to explain and illustrate theinvention without restricting its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

[0109]FIG. 1 is a schematic representation of the process of theinvention in its most basic form.

[0110]FIG. 2 is a graph of pressure-normalized pure-gas flux ofhydrogen, nitrogen and several light hydrocarbons as a function ofpressure for composite membranes having Hyflong AD60 selective layers.

[0111]FIG. 3 is a graph of calculated nitrogen/hydrocarbon selectivitybased on the pure gas data of FIG. 2.

[0112]FIG. 4 is a graph of calculated hydrogen/hydrocarbon selectivitybased on the pure gas data of FIG. 2.

[0113]FIG. 5 is a graph of pressure-normalized pure-gas flux ofhydrogen, nitrogen and several light hydrocarbons as a function ofpressure for composite membranes having Hyflon® AD80 selective layers.

[0114]FIG. 6 is a graph of calculated hydrogen/hydrocarbon selectivitybased on the pure gas data of FIG. 5.

[0115]FIG. 7 is a graph of calculated nitrogen/hydrocarbon selectivitybased on the pure gas data of FIG. 5.

[0116]FIG. 8 is a graph of pressure-normalized pure-gas flux ofhydrogen, nitrogen and several light hydrocarbons as a function ofpressure for composite membranes having Teflon(® AF 2400 selectivelayers.

[0117]FIG. 9 is a graph of calculated nitrogen/hydrocarbon selectivitybased on the pure gas data of FIG. 8.

[0118]FIG. 10 is a graph of calculated hydrogen/hydrocarbon selectivitybased on the pure gas data of FIG. 8.

[0119]FIG. 11 is a graph of pressure-normalized mixed-gas flux of carbondioxide, methane and propane as a function of pressure for compositemembranes having Hyflon® AD 60 selective layers.

[0120]FIG. 12 is a graph of mixed-gas carbon dioxide/methane and carbondioxide/propane selectivity based on the mixed gas data of FIG. 11.

[0121]FIG. 13 is a graph of pressure-normalized mixed-gas flux ofhydrogen and several light hydrocarbons as a function of pressure forcomposite membranes having Hyflon® AD 60 selective layers.

[0122]FIG. 14 is a graph of mixed-gas hydrogen/hydrocarbon selectivitiesbased on the mixed gas data of FIG. 13.

[0123]FIG. 15 is a graph of pressure-normalized mixed-gas flux ofmethane and n-butane as a function of n-butane concentration forcomposite membranes having Hyflon®t AD 60 selective layers.

[0124]FIG. 16 is a graph of mixed-gas methane/n-butane selectivity basedon the mixed gas data of FIG. 15.

[0125]FIG. 17 is a graph of pressure-normalized mixed-gas flux ofmethane and n-butane as a function of n-butane concentration forcomposite membranes having Hyflon® AD 80 selective layers.

[0126]FIG. 18 is a graph of mixed-gas methane/n-butane selectivity basedon the mixed gas data of FIG. 17.

[0127]FIG. 19 is a graph of pressure-normalized mixed-gas flux ofmethane and n-butane as a function of n-butane concentration forcomposite membranes having Teflon® AF2400 selective layers.

[0128]FIG. 20 is a graph of mixed-gas methane/n-butane selectivity basedon the mixed gas data of FIG. 19.

[0129]FIG. 21 is a graph of mixed-gas carbon dioxide/methane selectivityas a function of percent saturation of the gas mixture.

[0130]FIG. 22 is a graph of pressure-normalized mixed-gas flux of carbondioxide at 20° C. as a function of pressure for composite membraneshaving Hyflon® AD 60 selective layers.

[0131]FIG. 23 is a graph of mixed-gas carbon dioxide/methaneselectivitybased on the mixed-gas data of FIG. 22.

[0132]FIG. 24 is a graph of pressure-normalized mixed-gas flux of carbondioxide at -20° C. as a function of pressure for composite membraneshaving Hyflon® AD 60 selective layers.

[0133]FIG. 25 is a graph of mixed-gas carbon dioxide/methane selectivitybased on the mixed-gas data of FIG. 24.

[0134]FIG. 26 is a graph of mixed-gas carbon dioxide/methane selectivityas a function of percent saturation of the gas mixture, based on themixed-gas data of FIGS. 23 and 25.

[0135]FIG. 27 is a graph of pressure-normalized mixed-gas flux ofpropylene as a function of pressure for a spiral-wound module containingHyflon® AD 60 membranes.

[0136]FIG. 28 is a graph of mixed-gas propylene/propane selectivitybasedon the mixed-gas data of FIG. 27.

[0137]FIG. 29 is a graph of pressure-normalized mixed-gas fluxes ofpropylene and propane as a function of percent saturation of the gasmixture for a spiral-wound module containing Hyflon® AD 60 membranes.

[0138]FIG. 30 is a graph of pressure-normalized mixed-gas flux ofpropylene as a function of pressure for a spiral-wound module containingBPDA-TMPD polyimide membranes.

[0139]FIG. 31 is a graph of pressure-normalized mixed-gas flux ofpropane as a function of pressure for a spiral-wound module containingBPDA-TMPD polyimide membranes.

[0140]FIG. 32 is a graph of pressure-normalized mixed-gas flux ofnitrogen and dimethylethylamine as a function of dimethylethylamineconcentration for composite membranes having Hyflon®g AD 60 selectivelayers.

[0141]FIG. 33 is a graph of mixed-gas nitrogen/dimethylethylamineselectivity based on the mixed-gas data of FIG. 32.

[0142]FIG. 34 is a graph of pressure-normalized mixed-gas flux ofnitrogen and triethylamine as a function of triethylamine concentrationfor composite membranes having Hyflon® AD 60 selective layers.

[0143]FIG. 35 is a graph of mixed-gas nitrogen/triethylamine selectivitybased on the mixed-gas data of FIG. 34.

[0144]FIG. 36 is a schematic drawing of the process of the inventionapplied to treatment of refinery off-gas.

[0145]FIG. 37 is a schematic drawing of the process of the inventionapplied to treatment of an olefin/paraffin mixture from a petrochemicalmanufacturing plant.

DETAILED DESCRIPTION OF THE INVENTION

[0146] The term gas as used herein means a gas or a vapor.

[0147] The terms hydrocarbon and organic vapor or organic compound areused interchangeably herein, and include, but are not limited to,saturated and unsaturated compounds of hydrogen and carbon atoms instraight chain, branched chain and cyclic configurations, includingaromatic configurations, as well as compounds containing oxygen,hydrogen, halogen or other atoms.

[0148] The term C₂₊ hydrocarbon means a hydrocarbon having at least twocarbon atoms; the term C₃₊ hydrocarbon means a hydrocarbon having atleast three carbon atoms; and so on.

[0149] The terms light hydrocarbon and light olefin refer to moleculeshaving no more than about six carbon atoms.

[0150] The term heavier hydrocarbon means a C₃₊ hydrocarbon.

[0151] The terms two-step and multistep as used herein with regard to amembrane separation unit mean an arrangement of membrane modules orbanks of membrane modules connected together such that the residuestream from one module or bank of modules becomes the feedstream for thenext.

[0152] The terms two-stage and multistage as used herein with regard toa membrane separation unit mean an arrangement of membrane modules orbanks of membrane modules connected together such that the permeatestream from one module or bank of modules becomes the feedstream for thenext.

[0153] All percentages herein are by volume unless otherwise stated.

[0154] The invention is a process for separating a gas from a gasmixture containing an organic vapor or vapors. The gas mixture comprisesthe gas that is desired to be separated and other component orcomponents, of which at least one is an organic compound, typically aC₃₊ hydrocarbon vapor. The separation is carried out by running a streamof the gas mixture across a membrane that is selective for the desiredgas to be separated over the organic vapor. The process results,therefore, in a permeate stream enriched in the desired gas and depletedin the organic vapor, and a residue stream depleted in the desired gasand enriched in the organic vapor.

[0155] In a basic embodiment, the process of the invention includes thefollowing steps:

[0156] (a) bringing a gas mixture comprising a desired gas and anorganic compound, particularly a C₃₊ hydrocarbon vapor, into contactwith the feed side of a separation membrane having a feed side and apermeate side, the membrane having a selective layer comprising:

[0157] a polymer comprising repeating units having a fluorinated cyclicstructure of an at least 5-member ring, the polymer having a fractionalfree volume no greater than about 0.3;

[0158] (b) providing a driving force for transmembrane permeation;

[0159] (c) withdrawing from the permeate side a permeate stream enrichedin the desired gas compared to the gas mixture;

[0160] (d) withdrawing from the feed side a residue stream depleted inthe desired gas compared to the gas mixture.

[0161] The feed gas mixture to be separated often contains additionalcomponents, such as methane or ethylene, as well as C₃₊ hydrocarbons,that will be separated from the desired gas by the process. In thiscase, the process results in a permeate stream enriched in the desiredgas and depleted in both the C₃₊ hydrocarbon and the additionalcomponent compared with the feed gas mixture, and a residue streamdepleted in the desired gas and enriched in both the C₃₊ hydrocarbon andthe additional component compared with the feed gas mixture.

[0162] The process differs from processes previously available in theart in that:

[0163] (i) the membranes are able to maintain useful separationproperties in the presence of organic vapors, such as C₃₊ hydrocarbons,even at high levels in the gas, and

[0164] (ii) the membranes can recover from accidental exposure to liquidorganic compounds.

[0165] To provide these attributes, the process differs from previousgas/organic vapor separation processes in the nature of the membranethat is used. The membranes used in the process of the invention aremade from a glassy polymer, characterized by having repeating units of afluorinated, cyclic structure, the ring having at least five members.The polymer is further characterized by a fractional free volume nogreater than about 0.3 and preferably by a glass transition temperature,Tg, of at least about 100° C. Preferably, the polymer is perfluorinated.

[0166] These are not new polymers in themselves. In fact, generalpolymer formulations embracing those suitable for use in the inventionare described in patents dating back from the present day to the 1960s,for example, U.S. Pat. Nos. 3,308,107; 3,488,335; 3,865,845; 4,399,264;4,431,786; 4,565,855; 4,594,399; 4,754,009; 4,897,457; 4,910,276;5,021,602; 5,117,272; 5,268,411; 5,498,682; 5,510,406; 5,710,345;5,883,177; 5,962,612; and 6,040,419.

[0167] The ring structure within the repeat units may be aromatic ornon-aromatic, and may contain other atoms than carbon, such as oxygenatoms. Preferred polymers for the selective layer of the membrane areformed from fluorinated monomers of (i) dioxoles, which are five-memberrings of the form

[0168] that polymerize by opening of the double bond, or (ii)dioxolanes, similar five-member rings but without the double bond in themain ring, or (iii) aliphatic structures having an alkyl ether group,polymerizable into cyclic ether repeat units with five or six members inthe ring.

[0169] Not all polymers within the above structural definitions andpreferences are suitable for use as membrane selective layers in theinvention. For example, certain of the polymers and copolymers ofperfluoro-2,2-dimethyl-1,3-dioxole reported in U.S. Pat. No. 5,051,114have been shown to be susceptible to plasticization to the point ofswitching from being selective for nitrogen over hydrocarbons to beingselective for hydrocarbons over nitrogen as the hydrocarbon partialpressure increases. These polymers are, however, characterized by veryhigh fractional free volume within the polymer, typically above 0.3 .For example, a paper by A. Yu. Alentiev et al, “High transportparameters and free volume of perfluorodioxole copolymers”, JournalofMembrane Science, Vol. 126, pages 123-132 (1997) reports fractionalfree volumes of 0.32 and 0.37 for two grades ofperfluoro-2,2-dimethyl-1,3-dioxole copolymers (Table 1, page 125).Likewise, these polymers are of low density compared with otherpolymers, such as below about 1.8 g/cm³ and are unusually gas permeable,for instance exhibiting pure gas permeabilities as high as 1,000 Barreror more for oxygen and as high as 2,000 Barrer or more for hydrogen. Itis believed that polymers with denser chain packing, and thus lowerfractional free volume, higher density and lower permeability, are moreresistant to plasticization. Hence, the polymers used in the inventionto form the selective, discriminating layer of the membrane shouldpreferably be limited, in addition to the specific structurallimitations defined and discussed above, to those having a fractionalfree volume less than about 0.3.

[0170] In referring to fractional free volume (FFV), we mean the freevolume per unit volume of the polymer, defined and calculated as:

FFV=SFV/ν _(sp)

[0171] where SFV is the specific free volume, calculated as:

SFV=ν _(sp)−ν₀=ν_(sp)−1.3ν_(w)

[0172] and where:

[0173] ν_(sp) is the specific volume (cm³/g) of the polymer determinedfrom density or thermal expansion measurements,

[0174] ν₀ is the zero point volume at 0° K, and

[0175] ν_(w) is the van der Waals volume calculated using the groupcontribution method of Bondi, as described in D. W. van Krevelan,Properties of Polymers, 3^(rd) Edition, Elsevier, Amsterdam, 1990, pages71-76.

[0176] Expressed in terms of density, the selective layer polymersshould preferably have a density above about 1.8 g/cm³. Expressed interms of permeability, the selective layer polymers will generallyexhibit an oxygen permeability no higher than about 300 Barrer, moretypically no higher than about 100 Barrer, and a hydrogen permeabilityno higher than about 1,000 Barrer, more typically no higher than about500 Barrer.

[0177] Since the polymers used for the selective layer need to remainrigid and glassy during operation, they should also have glasstransition temperatures comfortably above temperatures to which they aretypically exposed during the process. Polymers with glass transitiontemperature above about 100° C. are preferred, and, subject also to theother requirements and preferences above, the higher the glasstransition temperature, in other words, the more rigid the polymer, themore preferred it is.

[0178] The polymers should preferably take amorphous, rather thancrystalline form, because crystalline polymers are typically essentiallyinsoluble and thus render membrane making difficult, as well asexhibiting low gas permeability.

[0179] As stated above, the polymers are fluorinated. More preferably,they have a fluorine:carbon ratio of atoms in the polymer of at leastabout 1:1, and most preferably, they are perfluorinated.

[0180] The polymers may be homopolymers of the repeating units offluorinated cyclic structures defined above. Optionally, they may becopolymers of such repeat units with other polymerizable repeat units.For preference, these other repeat units should be at least partiallyfluorinated, and most preferably heavily fluorinated or perfluorinated.A number of suitable materials are known, for example, fluorinatedethers, ethylene and propylene. Particularly when perfluorinated,homopolymers made from these materials, such as polytetrafluoroethylene(PTFE) and the like, are very resistant to plasticization. However, theytend to be crystalline or semi-crystalline and to have gaspermeabilities too low for any useful separation application. Asconstituents of copolymers with the fluorinated ring structures definedabove, however, they can produce materials that combine amorphousstructure, good permeability and good resistance to plasticization.Copolymers that include tetrafluoroethylene units are particularlypreferred. Other specific examples of copolymers that are suitable arepolyhexafluoropropylene and chlorofluoro ethylenes and propylene.

[0181] Specific most preferred materials are copolymersoftetrafluoroethylene with2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole having the structure:

[0182] where x and y represent the relative proportions of the dioxoleand the tetrafluoroethylene blocks, such that x+y =1.

[0183] Such materials are available commercially from Ausimont S.p.A.,of Milan, Italy under the trade name Hyflon® AD. Different grades areavailable varying in proportions of the dioxole and tetrafluoroethyleneunits, with fluorine:carbon ratios of between 1.5 and 2, depending onthe mix of repeat units. For example, grade Hyflon AD 60 contains a60:40 ratio of dioxole to tetrafluoroethylene units, has a fractionalfree volume of 0.23, a density of 1.93 g/cm³ and a glass transitiontemperature of 121° C., and grade Hyflon AD 80 contains an 80:20 ratioof dioxole to tetrafluoroethylene units, has a fractional free volume of0.23, a density of 1.92 g/cm³ and a glass transition temperature of 134°C.

[0184] A second highly preferred group of materials is the set ofpolyperfluoro (alkenyl vinyl ethers) including polyperfluoro (allylvinyl ether) and polyperfluoro (butenyl vinyl ether). A specific mostpreferred material of this type has the structure:

[0185] where n is a positive integer.

[0186] This material is available commercially from Asahi Glass Company,of Tokyo, Japan under the trade name Cytop®. Cytop has a fractional freevolume of 0.21, a density of 2.03 g/cm³, a glass transition temperatureof 108° C., and a fluorine:carbon ratio of 1.7.

[0187] In the alternative, the membranes are characterized in terms oftheir selectivity before and after exposure to liquid hydrocarbons.Specifically, the membranes have apost-exposure selectivity for thedesired gas over the gaseous hydrocarbon from which it is to beseparated, after exposure of the separation membrane to a liquidhydrocarbon, for example, toluene, and subsequent drying, that is atleast about 60%, 65% or even 70% of a pre-exposure selectivity for thedesired gas over the gaseous hydrocarbon, the pre- and post-exposureselectivities being measured with a test gas mixture of the samecomposition and under like conditions.

[0188] In this case, the selective layer is again made from an amorphousglassy polymer or copolymer with a fractional free volume no greaterthan about 0.3 and a glass transition temperature, Tg, of at least about100° C. The polymer is fluorinated, generally heavily fluorinated, bywhich we mean having a fluorine:carbon ratio of atoms in the polymer ofat least about 1:1. Preferably, the polymer is perfluorinated. In thiscase the polymer need not incorporate a cyclic structure.

[0189] A detailed discussion ofthis type of membrane material, membranesand processes using the membranes is given in copending patentapplication Ser. No. 09/574,303, which is incorporated herein byreference in its entirety.

[0190] A third group of materials that is believed to contain usefulselective layer materials is perfluorinated polyimides. Such materialshave been investigated for use as optical waveguides, and theirpreparation is described, for example, in S. Ando et al.,“Perfluorinated polymers for optical waveguides”, CHEMTECH, December,1994. To be usable as membrane materials, the polyimides have to becapable of being formed into continuous films. Thus, polyimides thatincorporate ether or other linkages that give some flexibility to themolecular structure are preferred. Particular examples are polymerscomprising repeat units prepared from the perfluorinated dianhydride1,4-bis(3,4-dicarboxytrifluorophenoxy) tetrafluorobenzene (10FEDA),which has the structure:

[0191] Diamines with which 10FEDA can be reacted to form polyamic acidsand hence polyimides include 4FMPD, which has the structure:

[0192] The resulting 10FEDA/4FMPD polyimide has the repeat unitstructure:

[0193] where n is a positive integer.

[0194] Yet further discussion of membrane materials is included incopending application Ser. No. 09/574,303, entitled “Gas SeparationUsing C₃₊ Hydrocarbon Resistant Membranes” incorporated herein byreference in its entirety.

[0195] The polymer chosen for the selective layer can be used to formfilms or membranes by any convenient technique known in the art, and maytake diverse forms. Because the polymers are glassy and rigid, anunsupported film, tube or fiber of the polymer may be usable inprinciple as a single-layer membrane. In this case, our preferred methodof manufacture is to prepare a solution of the polymer in aperfluorinated solvent and to cast the solution onto a glass plate or aremovable or non-removable backing web, according to general castingprocedures that are well known in the art. The resulting flat-sheetmembrane films may be dried under ambient conditions, at elevatedtemperature, or under vacuum as desired to produce thin film membranes.

[0196] Alternatively, the membrane may be manufactured in the form ofhollow fibers, the general methods for preparation of which arecopiously described in the literature, for example in U.S. Pat. No.3,798,185 to Skiens et al., incorporated herein by reference. However,such single-layer films will normally be too thick to yield acceptabletransmembrane flux, and in practice, the separation membrane usuallycomprises a very thin selective layer that forms part of a thickerstructure, such as an integral asymmetric membrane, comprising a denseregion that forms the separation membrane and a microporous supportregion. Such membranes were originally developed by Loeb and Sourirajan,and their preparation in flat sheet or hollow fiber form is nowconventional in the art and is described, for example, in U.S. Pat. No.3,133,132 to Loeb, and U.S. Pat. No. 4,230,463 to Henis and Tripodi.

[0197] As a further, and a preferred, alternative, the membrane may be acomposite membrane, that is, a membrane having multiple layers. Moderncomposite membranes typically comprise a highly permeable but relativelynon-selective support membrane, which provides mechanical strength,coated with a thin selective layer of another material that is primarilyresponsible for the separation properties. Typically, but notnecessarily, such a composite membrane is made by solution-casting thesupport membrane, then solution-coating the selective layer. Generalpreparation techniques for making composite membranes of this type arewell known, and are described, for example, in U.S. Pat. No. 4,243,701to Riley et al., incorporated herein by reference.

[0198] If the membrane is made in the form of a composite membrane, itis particularly preferred to use a fluorinated or perfluorinatedpolymer, such as polyvinylidene fluoride, to make the microporoussupport membrane. Again, the membrane may take flat-sheet, tube orhollow-fiber form. The most preferred support membranes are those withan asymmetric structure, which provides a smooth, comparatively densesurface on which to coat the selective layer. Support membranes arethemselves frequently cast onto a backing web of paper or fabric. As analternative to coating onto a support membrane, it is also possible tomake a composite membrane by solution-casting the polymer directly ontoa non-removable backing web, as mentioned above. In hollow-fiber form,multilayer composite membranes may be made by a coating procedure astaught, for example, in U.S. Pat. Nos. 4,863,761; 5,242,636; and5,156,888, or by using a double-capillary spinneret of the type taughtin U.S. Pat. Nos. 5,141,642 and 5,318,417.

[0199] The membrane may also include additional layers, such as a gutterlayer between the microporous support membrane and the selective layer,or a sealing layer on top of the selective layer. A gutter layergenerally has two purposes. The first is to coat the support with amaterial that seals small defects in the support surface, anditselfprovides a smooth, essentially defect-free surface onto which theselective layer may be coated. The second is to provide a layer ofhighly permeable material that can channel permeating molecules to therelatively widely spaced pores in the support layer. Preferred materialsfor the gutter layer are fluorinated or perfluorinated, to maintain highchemical resistance through the membrane structure, and of very highpermeability. Particularly preferred for the gutter layer, although theyare unsuitable for the selective layer, are the perfluorinated dioxolepolymers and copolymers of U.S. Pat. No. 5,051,114 referred to above,having fractional free volume greater than 0.3 and extraordinarily highpermeability, such as copolymers of perfluoro-2,2-dimethyl-1,3-dioxoleand tetrafluoroethylene, available commercially as Teflon® AF fromDuPont Fluoroproducts of Wilmington, Delaware. Such materials, or anyothers of good chemical resistance that provide protection for theselective layer without contributing significant resistance to gastransport, are also suitable as sealing layers.

[0200] Multiple selective layers may also be used.

[0201] The thickness of the selective layer or skin of the membranes canbe chosen according to the proposed use, but will generally be nothicker than 10 μm, and typically no thicker than 5 μm. It is preferredthat the selective layer be sufficiently thin that the membrane providea pressure-normalized hydrogen flux, as measured with pure hydrogen gasat 25° C., of at least about 100 GPU (where 1 GPU=1×10⁻⁶cm³(STP)/cm²·s·cmHg), more preferably at least about 200 GPU and mostpreferably at least about 400 GPU. In general, the membranes of theinvention provide transmembrane gas fluxes that are high compared withmembranes using conventional hydrogen-separating materials, such aspolyimides, cellulose acetate and polysulfone.

[0202] Once formed, the membranes exhibit a combination of goodmechanical properties, thermal stability, and high chemical resistance.The fluorocarbon polymers that form the selective layer are typicallyinsoluble except in perfluorinated solvents and are resistant to acids,alkalis, oils, low-molecular-weight esters, ethers and ketones,aliphatic and aromatic hydrocarbons, and oxidizing agents, making themsuitable for use not only in the presence of C₃₊ hydrocarbons, but inmany other hostile environments.

[0203] The membranes of the invention may be prepared in any knownmembrane form and housed in any convenient type of housing andseparation unit. We prefer to prepare the membranes in flat-sheet formand to house them in spiral-wound modules. However, flat-sheet membranesmay also be mounted in plate-and-frame modules or in any other way. Ifthe membranes are prepared in the form of hollow fibers or tubes, theymay be potted in cylindrical housings or otherwise.

[0204] The membrane separation unit comprises one or more membranemodules. The number of membrane modules required will vary according tothe volume of gas to be treated, the composition of the feed gas, thedesired compositions of the permeate and residue streams, the operatingpressure of the system, and the available membrane area per module.Systems may contain as few as one membrane module or as many as severalhundred or more. The modules may be housed individually in pressurevessels or multiple elements may be mounted together in a sealed housingof appropriate diameter and length.

[0205] The process of the invention in its most basic form is shown inFIG. 1. Referring to this figure, a feedstream, 1, containing a gasmixture including a desired gas and one or more organic compounds, ispassed into membrane separation unit 2 and flows across the feed side ofmembrane 3, which is characterized by having a selective layercomprising a polymer containing repeat units having a fluorinated cyclicstructure of an at least 5-member ring, the polymer having a fractionalfree volume no greater than about 0.3. Under apressure differencebetween the feed and permeate sides of the membrane, the desired gaspasses preferentially to the permeate side, and gas-enriched stream, 5,is withdrawn from the permeate side. The remaining gas-depleted,organic-enriched residue stream, 4, is withdrawn from the feed side.

[0206] The composition and pressure at which the feedstream is suppliedto the membrane modules varies depending on the source of the stream. Ifthe feed gas stream to be treated is at highpressure compared withatmospheric, such as 200 psia, 400 psia, 500 psia or above, theseparation may be effected simply by making use of this high pressure toprovide an adequate driving force and feed:permeate pressure ratio.Otherwise, a pressure difference can be provided by compressing the feedstream, by drawing a vacuum on the permeate side of the membrane, or acombination of both. Polymermembranes can typically withstand pressuredifferences between the feed and permeate side up to about 1,500-2000psi, so it might occasionally be necessary to let down the gas pressurebefore it can be fed to the membrane system.

[0207] An important consideration is the effect of hydrocarbons,particularly C₃₊ hydrocarbons, in the feed stream. Unlike prior artmembranes, the membranes of the invention can maintain usefulgas/hydrocarbon separation performance, in terms oftransmembrane gasflux and selectivity, when exposed to high concentrations of suchorganics, even when the gas mixture is close to saturation with thesecompounds. This is true with respect to a broad range of hydrocarbons,including paraffins, olefins, aromatics, such as benzene, toluene andxylenes (BTEX), alcohols and chlorinated compounds. These properties aredifferent from those reported in the literature for dioxole membranes,as well as obtained with prior art conventional membrane materials, suchas cellulose acetate, polysulfone, or polyimides that are notperfluorinated.

[0208] Even if condensation of organic liquid does accidentally occurfrom time to time, the membrane unit can generally be purged with, forexample, an inert gas such as nitrogen, and the membranes willfrequently continue thereafter to exhibit adequate gas/hydrocarbonselectivity properties.

[0209] In contrast, prior art membranes in commercial use are generallyplasticized and irreversibly damaged by exposure to C₃₊ hydrocarbonvapors at any significant concentration, such as more than about 10%,20% or 25%, or at more modest concentrations, such as less than 10%, forprolonged periods, and cannot withstand even fleeting exposure tocondensed organic liquids.

[0210] As a rough general guide, expressed as a concentration, the feedgas treated by the process of the invention may have a hydrocarbonscontent, including C₃₊ hydrocarbon vapors, of at least about 5%, 10%,15%, 20% or higher. Expressed in terms of partial pressure, the feedstream may often be acceptable with a partial pressure of C₃₊hydrocarbons of as high as 15 psia, 25 psia, 50 psia, 100 psia or more,assuming a gas temperature of ambient or above; and the residue streampartial pressure of the C₃₊ hydrocarbons together can often be as highas 50 psia, 100 psia, 150 psia or 200 psia, again assuming a temperatureof ambient or above. Expressed as the ratio of the feed pressure, P, tothe saturation vapor pressure, P_(sat), of the gas mixture, which is anapproximate measure of the activity of the gas, the feed gas may besupplied to the membrane separation step at pressure and temperatureconditions that result in the percentage P/P_(sat) being at least about25%, 30%, 50%, 60 %, 70% or higher. Methane and C₂ components, whichtend to have low boiling points, and to be less condensable and lessharmful in terms of their plasticizing ability, can generally be presentin any concentration.

[0211] Depending on the performance characteristics of the membrane, andthe operating parameters of the system, the process can be designed forvarying levels of gas purification and recovery. Single-stagegas-separation processes typically remove up to about 80-95% of thepreferentially permeating component from the feed stream and produce apermeate stream significantly more concentrated in that component thanthe feed gas. This degree of separation is adequate for manyapplications. If the residue stream requires further purification, itmay be passed to a second bank of modules for a second processing step.If the permeate stream requires further concentration, it may be passedto a second bank of modules for a second-stage treatment. Suchmultistage or multistep processes, and variants thereof, will befamiliar to those of skill in the art, who will appreciate that theprocess may be configured in many possible ways, including single-stage,multistage, multistep, or more complicated arrays of two or more unitsin series or cascade arrangements.

[0212] In light of their unusual and advantageous properties, themembranes and processes of the invention are useful for many separationapplications. Specific examples include, but are not limited toseparation of permanent gases, for example, nitrogen, oxygen, air, argonor hydrogen, from organics; separation of methane from C₃₊ organics;separation of carbon dioxide from organics; separation of light olefinsfrom other organics; and separation of isomers from one another, such an-butane from iso-butane.

[0213] Of particular importance, the membranes and processes of theinvention are useful for many applications where hydrogen is to beseparated from mixtures containing hydrogen and one or morehydrocarbons. In another aspect, therefore, the invention is a processfor treating refinery or petrochemical plant streams containing hydrogenand hydrocarbons, to separate hydrogen from the hydrocarbons.

[0214] The following list of applications of the invention in thisaspect is exemplary, but not limiting: separation of hydrogen frommethane and other light hydrocarbons in process and off-gas streamsfrom: hydrocrackers; hydrotreaters of various kinds, includinghydrodesulfurization units; coking reactors; catalytic reformers;catalytic crackers; specific isomerization, alkylation and dealkylationunits; steam reformers; hydrogenation and dehydrogenation processes; andsteam crackers for olefin production, as well as in streams frommanufacture of primary petrochemicals, chemical intermediates, fuels,polymers, agricultural chemicals and the like.

[0215] The treatment process of this invention, with respect to FIG. 1,involves running a refinery, chemical plant or the like stream, 1,containing a hydrogen/hydrocarbon mixture, typically including hydrogen,methane and C₃₊ hydrocarbons, across the feed side of a membraneseparation unit 2, containing a membrane characterized as before, 3,that is selectively permeable to the hydrogen over the methane and otherhydrocarbons in the stream. The hydrogen is concentrated in the permeatestream, 5; the residue stream, 4, is thus correspondingly depleted ofhydrogen.

[0216] The process differs from previous hydrogen/hydrocarbon separationprocesses in the nature of the membrane that is used. The membranes are,as described above, able to maintain useful separation properties in thepresence of organic vapors, particularly C₃₊ hydrocarbons, at highpartial pressure, and able to recover from accidental exposure to liquidhydrocarbons.

[0217] The scope of the invention in this aspect is not intended to belimited to any particular gas streams, but to encompass any situationwhere a gas stream containing hydrogen and hydrocarbon gas is to beseparated. The composition of the gas may vary widely, from a mixturethat contains minor amounts ofhydrogen in admixture with varioushydrocarbon components, including relatively heavy hydrocarbons, such asC₅-C₈ hydrocarbons or heavier, to a mixture of mostly hydrogen, such as80% hydrogen, 90% hydrogen or above, with methane and other very lightcomponents. Typical examples of compositions and pressures of feed gasessuitable for treatment by the process of the invention, include, but arenot limited to, mixtures of hydrogen with methane and C₂-C₈ paraffinsand olefins having a C₃₊ hydrocarbon content of as much as 15-20% ormore at a total feed pressure of 400 psia; mixtures of hydrogen andmethane of any composition and pressure; and mixtures of hydrogen withC₁-C₄ paraffins having a total hydrocarbon content of as much as 60% ormore at a total feed pressure of 500 psia.

[0218] The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbon,for hydrogen over methane of at least about 10, for hydrogen overpropane of at least about 50, and for hydrogen over n-butane of at leastabout 100. Frequently, the hydrogen/methane selectivity achieved is 20or more, even in the presence of significant concentrations of C₃₊hydrocarbons.

[0219] Applications range from those treating very large streams, suchas separation of hydrogen/light hydrocarbon mixtures in ethylene plantcold trains, to those handling much smaller streams, including recoveryof hydrogen from vent streams generated by hydrogen reduction processes.

[0220] A particularly attractive use of the process is to recoverhydrogen from streams containing less than about 40% hydrogen, and richin C₃₊ hydrocarbons, for which PSA or cryogenic condensation is noteconomically attractive. Such streams typically have flow rates below 50MMscfd, and prior to the availability of the present process wereusually not separated, but were used as fuel. This is a waste ofvaluable resources, as the difference between the fuel and chemicalvalues of such a gas stream can be as much as $2/1,000 scf of gas. Notonly is it a waste of resources, however, but in some cases the quantityof fuel-grade gas generated by unit operations in the plant is so greatthat the plant becomes bottlenecked by over supply of fuel gas.

[0221] The process of the invention can be used to produce ahydrogen-rich permeate stream, containing, for example, 90% hydrogen. Astream of such composition may be recompressed and used in otherrefinery unit operations, or subjected to further treatment to yieldhigh purity hydrogen as required. The hydrocarbon-rich residue streammay be piped to the fuel header, thereby reducing the volume of fuel gasproduced, or sent for LPG recovery, for example. In such a process,recovery of 50%, 60%, 70%, 80% or more of the hydrogen originallypresent in the waste stream is possible.

[0222] A second specific attractive application is hydrogen and olefinrecovery from fluid catalytic cracking (FCC) off-gas. The FCC stream isthe largest hydrogen-containing off-gas produced in a refinery. FCCoff-gas streams are typically in the range 10 to 50 MMscfd and contain10-20% hydrogen at 100-250 psig. The membrane process of the inventioncan be used to recover both hydrogen and hydrocarbons from thesestreams. The hydrogen product, typically containing 80-90% hydrogen, canbe used effectively in many applications, such as low-pressurehydrotreating. The hydrocarbon-rich residue can be used as fuel, or canbe sent for olefin recovery from the hydrocarbon mixture by cryogenicdistillation or the like.

[0223] A third specific application is the separation of hydrogen fromethylene steam cracker product gas. Low molecular weight olefins,particularly ethylene and propylene, are typically made by crackingethane or propane with steam. The gas mixture leaving the cracker is amixture of hydrogen, methane, hydrocarbons and carbon dioxide. Aftercarbon dioxide has been removed, for example by absorption into an amineor sodium hydroxide solution, the mixture typically has a composition ofabout 20% hydrogen, 25-30% methane/ethane, 40-45% ethylene/propylene and5-10% propane/butane. The process of the invention can be used toseparate hydrogen from this mixture, either before or after the productolefins have been removed by cooling/condensation/fractionation.

[0224] A final exemplary set of applications is in the treatment ofgases circulating in a reactor loop. Many operations carried out inrefineries and petrochemical plants involve feeding ahydrocarbon/hydrogen stream to a reactor, withdrawing a reactor effluentstream of different hydrocarbon/hydrogen composition, phase separatingthe effluent into liquid and vapor portions, and recirculating part ofthe vapor stream to the reactor, so as to reuse unreacted hydrogen. Suchloop operations are found, for example, in the hydrotreater,hydrocracker, and catalytic reformer sections of most modern refineries,as well as in isomerization reactors and hydrodealkylation units.

[0225] In addition to hydrogen, the overhead vapor from the phaseseparation usually contains light hydrocarbons, particularly methane andethane. In a closed recycle loop, these components build up, change thereactor equilibrium conditions and can lead to catalyst degradation andreduced product yield. This build-up of undesirable contaminants isusually controlled by purging a part of the vapor stream from the loop.Such apurge operation is unselective however, and, since the purgestream may contain as much as 80 vol % or more hydrogen, multiplevolumes of hydrogen can be lost from the loop for every volume ofcontaminant that is purged.

[0226] The process of the invention may be used to provide a selectivepurge capability. The overhead vapor from the phase separation step, ora portion thereof, is treated to provide a purified hydrogen permeatestream, which may be recirculated in the reactor loop, and ahydrocarbon-rich, hydrogen-depleted residue stream, which forms thepurge stream. In this way purging can be carried out with reduced lossof hydrogen with the purged gas. Such reactor loops in which theinvention can be used are found in hydrocracking, hydrotreating,catalytic reforming and hydrogenation, for example.

[0227] The invention has been described in this aspect as it relates tothe separation of hydrogen from hydrocarbon-containing gas mixtures.Processes that concern the separation of other permanent gases from gasmixtures containing hydrocarbons are also possible using the membranesas characterized above. Specific examples include, but are not limitedto, separation of nitrogen, oxygen or air from methane, ethylene orother organics; and separation of argon from ethylene.

[0228] Such a process is useful, for example, in separating nitrogenfrom methane to treat natural gas that is out of specification bycontaining excess nitrogen. In this case, the natural gas stream to betreated contains methane in addition to C₃₊ hydrocarbons. The process isalso useful for treating gas mixtures in which nitrogen is to beseparated from ethylene, such as occur in polyolefin manufacturing. Yetanother use is to treat off-gases from numerous industrial processesthat produce waste streams containing organic vapors in air or nitrogen,such as arise when organic solvents are used in coating, spraying,cleaning, painting, or printing applications of all kinds, from organicliquid storage tank vents, from chemical manufacturing, or from foundrycold boxes using organic catalysts for metal casting. In this case,diverse organic vapors may be present in the stream, for example,halogenated solvents, alkyl amines, ketones or alcohols.

[0229] The process of the invention can provide a selectivity, in gasmixtures, for nitrogen over methane of as high as 2, 2.5 or even 3.Although these numbers seem small, they are remarkable, in that fewprior art membrane materials offer any selectivity at all for nitrogenover methane. For example, polysulfone, cellulose acetate andpolycarbonate all have nitrogen/methane selectivity of only about 1 orbelow, that is, they often exhibit slight methane/nitrogen selectivity.Non-perfluorinated polyimides, the best group of prior art materials inthis regard, offer typical nitrogen/methane selectivity, even asmeasured with pure gases, only in the range between 1 and 2.3.

[0230] The process of the invention can also provide exceptionalselectivity, in gas mixtures, for nitrogen over ethylene of as high as4, 5 or above. This performance is again unusual compared with othermaterials.

[0231] As it relates to separation of nitrogen from more complex organicmolecules, the process of the invention can provide much highermixed-gas selectivities, such as 20, 40, 50, 100 or higher, depending onthe nature of the organic compound and the process conditions.

[0232] A large number of chemical products are produced by catalyticoxidation of an appropriate organic feedstock. For example, ethyleneoxide is made by oxidation of ethylene, as are acetaldehyde, vinylacetate and vinyl chloride; propylene oxide and acrylonitrile areproduced by oxidation of propylene; benzoic acid by oxidation oftoluene; and caprolactam by oxidation of cyclohexane. Such oxidationprocesses operate in a loop, with modest conversion per pass, so thatlarge amounts of unreacted organic feedstock are recirculated back tothe reaction zone at each pass. The processes often use a feed ofoxygen-enriched air or high-purity oxygen as the oxygen source, leadingto a build-up of unreacted argon, which enters with the feed oxygen, inthe reactor loop. The process of the invention can be used toselectively purge argon from the loop, while retaining the ethylene,propylene or other organic feedstock for recycle to the process.

[0233] In this respect, the process of the invention typically providesa selectivity, in gas mixtures, for argon over ethylene of as high as 4,5, 6, 7 or above. These are again very unusual and advantageousproperties.

[0234] In yet another aspect, the invention is aprocess for separatingcarbon dioxide from methane and/or other hydrocarbons. Such a mixturemight be encountered during the processing of natural gas, of associatedgas from oil wells, or of certain petrochemical streams, for example.

[0235] For natural gas to be accepted into the pipeline, it mustnormally contain no more than 4% carbon dioxide. As mentioned above,much raw natural gas is out of specification in this regard, as well asbeing too rich in C₃₊ hydrocarbons content. Sour gas streams also ariseas a result of oil extraction by miscible flood operations. In theseoperations, carbon dioxide is injected into the ground at the peripheryof an oil reservoir. The gas dissolves in the oil left in the pore spaceof the formation and lowers its viscosity. The resulting mixture is thenpushed by water or gas pressure to the extraction wells. Initially theassociated gas extracted with the oil is rich in methane, but over timethe methane concentration falls and the carbon dioxide concentrationrises, to as much as 80 or 90%. The mixture extracted from the wells isseparated into recovered oil, produced water, carbon dioxide forreinjection, and condensed hydrocarbon liquids (NGL). Separation of thecarbon dioxide from the methane and other hydrocarbons in the associatedgas is important for the process to be economically sound.

[0236] In the aspect of carbon dioxide/organic separations, the processof the invention, again with reference to FIG. 1, involves running astream containing carbon dioxide, 1, across the feed side of a membraneseparation unit, 2, containing a membrane as defined above, 3, that isselectively permeable to the carbon dioxide over methane and otherhydrocarbons in the stream. The carbon dioxide is concentrated in thepermeate stream, 5; the residue stream, 4, is thus correspondinglydepleted of carbon dioxide.

[0237] The process differs from previous carbon dioxide/methaneseparation processes in the nature of the membrane that is used. Themembranes are, as described above, able to maintain useful separationproperties in the presence of C₃₊ hydrocarbon vapor at high partialpressure, and able to recover from accidental exposure to liquidhydrocarbons. The membranes are also able to withstand high partialpressures of carbon dioxide.

[0238] The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbon,for carbon dioxide over methane of at least about 5, even at high carbondioxide activity. Frequently, the carbon dioxide/methane selectivityachieved is 10 or more, and may be as much as 15 or more, even in thepresence of significant concentrations of C₂₊ hydrocarbons.

[0239] In a different aspect, the invention is a process for separatingnot inorganic gases or vapors from organic gases or vapors, but organicgases or vapors from one another. In this aspect, the process of theinvention, again with reference to FIG. 1, involves running a streamcontaining a mixture of organic compounds, 1, across the feed side of amembrane separation unit, 2, containing a membrane as defined above, 3,that is selectively permeable to a first organic compound over a secondorganic compound in the stream. The first organic component isconcentrated in the permeate stream, 5; the residue stream, 4, is thuscorrespondingly depleted of that component.

[0240] The process differs from previous hydrocarbon/hydrocarbonseparation processes in the nature of the membrane that is used. Themembranes are, as described above, more resistant to plasticization byhydrocarbons than prior art membranes, and are able to recover fromaccidental exposure to liquid hydrocarbons.

[0241] The process of the invention may be used for diverse separationsof organic components, including, but not limited to, separation ofmethane from C₃₊ hydrocarbon vapors, separation of olefins fromparafins; and separation of isomers, such as n-butane from iso-butane.

[0242] In this aspect, the invention can be used in natural gasprocessing, for example, to lower the Btu value and dewpoint of gas thatinitially contains excess C₃₊ hydrocarbons. The invention may also beused to remove the lightest hydrocarbons, specifically methane andethane, from process streams, to prevent their build up in a reactorloop, for example.

[0243] The process of the invention typically provides a selectivity formethane over C₃₊ hydrocarbons, such as propane, butane or heavier, inmixtures containing multiple hydrocarbons including the C₃₊ hydrocarbon,of at least about 4 or 5, and in many cases, at least about 8.Frequently, the selectivity achieved is 10 or more, and may be as muchas 15 or more, even in the presence of significant concentrations of C₃₊hydrocarbons.

[0244] As it relates to the separation of olefins from paraffins, theinvention is particularly useful for separating propylene from propane.Such mixtures are found as olefin manufacturing effluent streams, and invarious petrochemical plant streams, for example.

[0245] The process involves running a stream comprising propylene andpropane across the feed side of a membrane that is selectively permeableto propylene. The propylene is concentrated in the permeate stream; theresidue stream is thus correspondingly depleted of propylene.

[0246] The process typically provides a propylene/propane selectivity ofat least about 2.5, more preferably at least about 3, which can besustained, even with streams composed entirely of C₃₊ hydrocarbons, overa range of pressures.

[0247] Since the membranes used in the invention are selective for botholefins and hydrogen over paraffins, the membrane separation step may beused, where both are present, to produce a permeate enriched in botholefins and hydrogen, leaving a residue stream enriched in paraffins.The olefins in the permeate stream may then be separated from thehydrogen to deliver product streams of each.

[0248] Optionally, the processes of the invention already discussed mayinclude other separation steps used in conjunction with the definedmembrane separation process. Examples of such separation steps includeadsorption, absorption, condensation, and distillation. The otherseparation steps may be carried out upstream, downstream or both of themembrane separation step, that is, with reference to FIG. 1 on any ofstreams 1, 4 and 5.

[0249] As non-limiting examples, streams may be filtered to separate outentrained oil or water droplets, passed through a glycol absorption unitfor dehydration, subjected to amine scrubbing to remove hydrogen sulfideor carbon dioxide, or cooled to condense out high boiling components.

[0250] As a specific illustration, processes that include the membraneseparation step combined with a pressure swing adsorption (PSA) step arewithin the scope of the invention. Details of the operation of PSA unitsare well documented in the art, and do not require lengthy descriptionhere. It is also well known to combine PSA with membrane separation, asis disclosed, for example, in U.S. Pat. No. 6,011,192.

[0251] PSA is often used, for example, to produce high-purity hydrogenfrom mixed streams containing light hydrocarbons with 60% or morehydrogen. The process generally operates at about 80% recovery; in otherwords, as much as 20% or more of the hydrogen content of the feed islost with the tail gas produced when the PSA beds are regenerated.

[0252] Membrane processes as described herein can be used in conjunctionwith adsorption processes, and particularly PSA, to improve hydrogenrecovery in several ways. For example, the hydrogen content in low-gradefuel gas, containing typically only 30-40% hydrogen, can be upgraded torender the gas suitable for hydrogen recovery by PSA. With reference yetagain to FIG. 1, a low-grade stream is passed as feed stream, 1, to themembrane separation unit, 2, containing a membrane as defined above, 3,that is selectively permeable to hydrogen over hydrocarbons. Thehydrogen is concentrated to, for example, 60-70% hydrogen in thepermeate stream, 5. At this composition, stream 5 is optionally, butpreferably, compressed, such as to a few hundred psia, and passed asfeed to a PSA unit to produce high-quality hydrogen. The residue stream,4, correspondingly depleted of hydrogen, may be sent to the fuel gasline.

[0253] Alternatively or additionally, a membrane step can be used torecover hydrogen currently lost with the tail gas when the PSA beds areregenerated. In this embodiment, a PSA tail gas stream is passed as feedstream, 1, to the membrane separation unit, 2, containing a membrane asdefined above, 3, that is selectively permeable to hydrogen overhydrocarbons. The hydrogen is concentrated to, for example, 60-70%hydrogen in the permeate stream, 5. At this composition, stream 5 may becompressed, if necessary, and returned as part of the feed to the PSAunit. The residue stream, 4, correspondingly depleted of hydrogen, maybe sent to the fuel gas line.

[0254] Thus, the processes of the invention as they relate tocombinations of membrane separation with PSA are especially useful forall of the hydrogen exemplary applications, such as upgrading of wastestreams from all sources previously limited to use as fuel gas, hydrogenand olefin recovery from fluid catalytic cracking (FCC) off-gas,separation of hydrogen from ethylene steam cracker product gas, andtreatment of purge gases from reactor loops.

[0255] The processes of the invention as they relate to combinations ofmembrane separation with PSA are also useful for all of the other typesof gas/organic vapor separations mentioned herein, including, but notlimited to, natural gas treatment by carbon dioxide or nitrogen removal,treatment of petrochemical plant gas streams, treatment of streamscontaining organic vapors in air, and so on.

[0256] The invention is now illustrated in further detail by specificexamples. These examples are intended to further clarify the invention,and are not intended to limit the scope in any way.

EXAMPLES Example 1 Membrane Making

[0257] Asymmetric, microporous poly(vinylidene fluoride) [PVDF] supportmembranes were prepared. Composite membranes were prepared using thefollowing coating solutions:

[0258] 1 wt % copolymer solution of 40% tetrafluoroethylene/60%2,2,4-trifluoro-5-trifluorometoxy-1,3-dioxole (Hyflon(® AD 60),(Ausimont, Italy), in a perfluorinated solvent (Fluorinert FC-84), (3M,St. Paul, Minn.).

[0259] 1 wt % copolymer solution of20%tetrafluoroethylene/80%2,2,4-trifluoro-5-trifluorometoxy-1,3-dioxole (Hyflon® AD80), (Ausimont,Italy), in FC-84 solvent.

[0260] 1 wt % polyperfluoro (alkenyl vinyl ether) (Cytop®), (AsahiGlass, Japan), in FC-84 solvent.

[0261] The support membranes were dip-coated in a solution of one of thethree selective polymer solutions at 1 ft/min coating speed, then driedin an oven at 60° C. for 10 minutes. The resulting membranes had aselective layer thickness ranging from 0.2-0.5 μm. Samples of eachfinished composite membrane were cut into 12.6 cm² stamps and tested ina permeation test-cell apparatus with pure gases at 35° C. feedtemperature and 65 psia feed pressure. During each test, the feed,permeate, and residue compositions were analyzed by gas chromatography(GC). The gas fluxes of the membranes were measured, and theselectivities were calculated. Table 1 summarizes the fluxes and Table 2summarizes the selectivities of the composite membranes, calculated asthe ratio of the pure gas fluxes. TABLE 1 Pure-Gas Pressure-NormalizedFlux (GPU) Gas Hyflon ® AD60 Hyflon ® AD80 Cytop ® Nitrogen 52 184 34Oxygen 180 574 130 Helium 1,360 1,850 1,270 Hydrogen 790 2,040 620 Argon85.4 289 56 Carbon Dioxide 433 — 300 Methane 17.6 65.8 11 Ethane 4.518.8 3 Ethylene 9.8 35.9 5.7 Propane 1.1 — 3.4 Propylene 5.1 25.6 — CF₄0.94 3.38 0.48 NF₃ 10.3 38.8 5.7

[0262] TABLE 2 Selectivity (−) Gas Pair Hyflon ® AD60 Hyflon ® AD80Cytop ® N₂/CF₄ 55 58 71 O₂/N₂ 3.5 3.1 3.8 N₂/CH₄ 2.9 2.8 3.2 He/H₂ 1.70.91 2.0 Ar/CH₄ 4.8 4.4 5.3 Ar/C₂H₄ 8.7 8.0 9.7 CO₂/CH₄ 26 — 28 H₂/CH₄45 31 59 N₂/C₂H₄ 5.3 5.1 6.0 N₂/C₂H₆ 10 7.2 —

Example 2 Mixed-Gas Argon/Ethylene Permeation Properties

[0263] Membranes were prepared and membrane stamps were subjected topermeation experiments using the same general procedure as in Example 1.The temperature was 23° C., the feed pressure was 165 psia, and the feedgas mixture contained approximately 9% argon, 67% methane and 24%ethylene. The pressure-normalized fluxes of argon and ethylene weremeasured, and the argon/ethylene selectivities were calculated. Theresults are shown in Table 3. TABLE 3 Mixed-Gas Pressure-Normalized Flux(GPU) Ar/C₂H₄ Membrane Ar C₂H₄ Selectivity (−) Hyflon ® AD60 88 11.9 7.4Hyflon ® AD80 265 42.7 6.2 Cytop ® 51 5.8 8.8

Example 3 Mixed-Gas Nitrogen/Ethylene Permeation Properties

[0264] Membranes were prepared and membrane stamps were subjected topermeation experiments using the same general procedure as in Example 1.The temperature was 23° C., the feed pressure was 165 psia, and the feedgas mixture contained 80% nitrogen and 20% ethylene. Thepressure-normalized fluxes of nitrogen and ethylene were measured, andthe nitrogen/ethylene selectivities were calculated. The results areshown in Table 4. TABLE 4 Mixed-Gas Pressure-Normalized Flux (GPU)N₂/C₂H₄ Membrane N₂ C₂H₄ Selectivity (−) Hyflon ® AD60 53 11 4.8Hyflon ® AD80 184 41.8 4.4 Cytop ® 31 5.3 5.8

Example 4 Mixed-Gas Carbon Dioxide/Methane Permeation Properties

[0265] Membranes were prepared and membrane stamps were subjected topermeation experiments using the same general procedure as in Example 1.The temperature was 22° C., the feed pressure was 115 psia, and the feedgas mixture contained 65% carbon dioxide, 25% methane and 10% propane.The pressure-normalized fluxes of carbon dioxide and methane weremeasured, and the carbon dioxide/methane selectivities were calculated.The results are shown in Table 5. TABLE 5 Mixed-Gas Pressure-NormalizedFlux (GPU) CO₂/CH₄ Membrane CO₂ CH₄ Selectivity (−) Hyflon ® AD60 460 2717 Hyflon ® AD80 1,620 125 13 Cytop ® 128 5.8 22

Example 5 Binary-Mixed-Gas Carbon Dioxide/Methane Permeation Properties

[0266] A Hyflon® AD60 membrane was prepared and subjected to permeationexperiments using the same general procedure as in Example 1. Thetemperatures ranged from -20 to 20° C., the feed pressures ranged from115 to 415 psia, and the feed gas mixture contained 70% carbon dioxideand 30% methane. The pressure-normalized fluxes of carbon dioxide andmethane were measured, and the carbon dioxide/methane selectivities werecalculated. The results are shown in Table 6. TABLE 6 Mixed-GasPressure-Normalized Temperature Pressure Flux (GPU) CO₂/CH₄ (° C.)(psia) CO₂ CH₄ Selectivity (−) 20 115 89 5.2 17 −20 115 92 2.6 36 −20215 113 3.8 29 −20 315 279 13 21 −20 415 1,420 167 8.5

[0267] As can be seen from the table, the membranes retained usefulcarbon dioxide/methane selectivities over the test range. At −20° C.,the saturation vapor pressure of carbon dioxide is 285 psia. Under theextreme conditions of low temperature (−20° C.) combined with highpressure (415 psia) of the test, carbon dioxide partial pressure reached290 psia, i.e., saturation. Even when the gas mixture was saturated withcarbon dioxide, the membranes withstood plasticization by carbon dioxidewell enough to retain the carbon dioxide/methane selectivity at a usablelevel.

Example 6 Solvent Resistance of Hyflon® AD60 compared to Polysulfone

[0268] Experiments were carried out to determine the stability of aHyflon® AD60 membrane in the presence of hydrocarbon solvents. Samplesof a Hyflon® AD60 membrane were tested in a permeation test-cell as inExample 1. The fluxes were measured and the selectivities calculated.The membrane stamps were then immersed in liquid toluene or hexane.After one week, the membranes were removed from the hydrocarbon liquid,dried at ambient temperature, and retested in the gas permeationtest-cell. A polysulfone (PSF) asymmetric membrane, typically used inhydrogen separation processes, was also tested for comparison. Thepermeation properties of the Hyflong AD60 and polysulfone membranesbefore and after exposure to the hydrocarbon solvent are summarized inTable 7. TABLE 7 Initial Initial Post-Toluene Post-Toluene FluxSelectivity Flux Selectivity (GPU) (−) (GPU) (−) Membrane N₂ H₂ O₂/N₂H₂/CH₄ N₂ H₂ O₂/N₂ H₂/CH₄ Hyflon ® 30 350 3.1 25 41 477 3.1 26 PSF 1.2 —5.6 — Dissolved Initial Initial Post-Hexane Post-Hexane Flux SelectivityFlux Selectivity (GPU) (−) (GPU) (−) Membrane N₂ H₂ O₂/N₂ H₂/CH₄ N₂ H₂O₂/N₂ H₂/CH₄ Hyflon ® 31 350 3.0 24 41 480 3.1 27 PSF 0.6 50 6.8 99 1.687 5.9 48

[0269] As can be seen, the polysulfone membranes could not withstandexposure to toluene, and their hydrogen/methane selectivity declined byhalf after exposure to hexane. In contrast, the dioxole copolymerHyflon® membranes, although they exhibited higher fluxes for all gasesfor which they were tested after soaking in liquid hydrocarbons,retained their hydrogen/methane selectivity.

Examples 7-10 Comparative Examples with Teflon® AF 2400 CompositeMembranes—Not in Accordance with the Invention Example 7 Membrane Making

[0270] Asymmetric, microporous poly(vinylidene fluoride) [PVDF] supportmembranes were prepared. Composite membranes were prepared bydip-coating the support membranes three times in a solution of 1 wt %2,2-bistrifluoromethyl-4,5-difluoro- 1,3-dioxole/tetrafluoroethylenecopolymer [Teflon® AF2400] solution in FC-84 solvent at 1 ft/min coatingspeed, then dried in an oven at 60° C. for 10 minutes. The resultingmembranes had a selective layer thickness of 4 μm. Samples of eachfinished composite membrane were cut into 12.6 cm² stamps and tested ina permeation test-cell apparatus with pure oxygen and nitrogen at 22° C.feed temperature and 65 psia feed pressure. During each test, the feed,permeate, and residue compositions were analyzed by gas chromatography(GC). The gas fluxes were measured, and the selectivities werecalculated. Table 8 summarizes the pressure-normalized fluxes andselectivities of the composite Teflon® AF membranes. TABLE 8 Mixed-GasPressure-Normalized Flux (GPU) Selectivity (−) N₂ O₂ O₂/N₂ 185 353 1.9

Example 8 Mixed-Gas Argon/Ethylene Permeation Properties

[0271] Membranes were prepared and subjected to permeation experimentsusing the same general procedure as in Example 7. The temperature was22° C., the feed pressure was 165 psia, and the feed gas mixture wasapproximately 8% argon, 65% methane and 27% ethylene. Thepressure-normalized fluxes of the gases were measured, and theselectivities were calculated. The results are shown in Table 9. TABLE 9Mixed-Gas Pressure-Normalized Flux (GPU) Selectivity (−) Ar CH₄ C₂H₄Ar/CH₄ Ar/C₂H₄ 232 159 158 1.5 1.5

[0272] As can be seen, the membranes were only slightly selective forargon over ethylene. In contrast, Example 2 showed that the membranes ofthe invention had exceptionally high argon/ethylene selectivities in therange about 6 to 9.

Example 9 Mixed-Gas Nitrogen/Ethylene Permeation Properties

[0273] Membranes were prepared and subjected to permeation experimentsusing the same general procedure as in Example 7. The temperature was22° C., the feed pressure was 165 psia, and the feed gas mixture was 80%nitrogen and 20% ethylene. The pressure-normalized fluxes of nitrogenand ethylene were measured, and the nitrogen/ethylene selectivities werecalculated. The results are shown in Table 10. TABLE 10 Mixed-GasPressure-Normalized Flux Selectivity (−) N₂ C₂H₄ N₂/C₂H₄ 177 159 1.1

[0274] The membrane was essentially unselective for nitrogen overethylene. In contrast, Example 3 showed selectivities of about 4 to 6for nitrogen over ethylene for the membranes of the invention.

Example 10 Mixed-Gas Carbon Dioxide/Methane Permeation Properties

[0275] Membranes were prepared and subjected to permeation experimentsusing the same general procedure as in Example 7. The temperature was22° C., the feed pressure was 115 psia, and the feed gas mixture was 64%carbon dioxide, 25% methane and 11% propane. The pressure-normalizedfluxes of the gases were measured, and the selectivities werecalculated. The results are shown in Table 11. TABLE 11 Mixed-GasPressure-Normalized Flux (GPU) Selectivity (−) CO₂ CH₄ C₃H₈ CO₂/CH₄CO₂/C₃H₈ 831 175 95.7 4.8 8.7

[0276] In this case, the carbon dioxide/methane selectivity was only4.8, compared with 13-22 in experiments under similar conditions withthe membranes of the invention reported in Example 4.

Examples 11-13 Comparison of Pure-Gas Permeation Properties with Hyflon®AD and Teflon® AF2400 Membranes Example 11 Hyflon® AD60 Pure-GasPermeation Properties

[0277] Hyflon® AD60 membranes were prepared as in Example 1, exceptusing a poly(etherimide) support layer. The resulting membranes weretested as in Example 1 with pure hydrogen, nitrogen, methane, ethane,propane, and n-butane at 35° C. at feed pressures ranging from 35 to 165psia. The n-butane was tested only at 32 psia, which is nearly 70% ofthe saturation vapor pressure of n-butane at 35° C. The measuredpressure-normalized gas fluxes are shown graphically in FIG. 2. Thecalculated nitrogen/hydrocarbon selectivities are shown graphically inFIG. 3, and the calculated hydrogen/hydrocarbon selectivities are shownin FIG. 4.

[0278] As can be seen in FIG. 2, the hydrogen, nitrogen, and methanefluxes remained nearly constant across the range of pressures. Theethane flux increased from 6.9 GPU at 65 psia to 12.6 GPU at 165 psia,and the propane flux increased from 1.4 GPU at 35 psia to 3.9 GPU at 125psia, which is about 70% of the saturation vapor pressure (180 psia) ofpropane at 35° C. As shown in FIG. 3, the nitrogen/methane selectivityremained constant at approximately 2.3 across the range of pressures.The nitrogen/ethane selectivity decreased from 8.2 at 65 psia to 4.5 at165 psia, and the nitrogen/propane selectivity decreased from 42 at 35psia to 30 at 125 psia. Although the nitrogen/propane selectivitydecreased over the pressure range, the membrane remained nitrogenselective, at a useful selectivity, over the entire pressure range up tonear-saturation.

[0279] As shown in FIG. 4, the hydrogen/methane selectivity remainedconstant at approximately 29 across the range of pressures. Thehydrogen/ethane selectivity decreased slightly from 97 at 65 psia to 83at 115 psia, then decreased further to 57 at 165 psia. Thehydrogen/propane selectivity was 230 at 115 psia.

Example 12 Hyflon® AD80 Pure-Gas Permeation Properties

[0280] Hyflon® AD80 membranes were prepared as in Example 1, exceptusing a poly(etherimide) support layer. The resulting membranes weretested as in Example 1 with pure hydrogen, nitrogen, methane, ethane,propane, and n-butane at 35° C. at feed pressures ranging from 35 to 165psia. The n-butane was tested only at 32 psia, which is nearly 70% ofthe saturation vapor pressure of n-butane at 35° C. The measuredpressure-normalized gas fluxes are shown graphically in FIG. 5. Thecalculated hydrogen/hydrocarbon selectivities are shown graphically inFIG. 6 and the calculated nitrogen/hydrocarbon selectivities are shownin FIG. 7.

[0281] As can be seen in FIG. 5, the hydrogen, nitrogen, methane andethane fluxes remained nearly constant across the range of pressures.The propane flux increased from 3 GPU at 35 psia to 6.6 GPU at 120 psia.As shown in FIG. 6, the hydrogen/methane and hydrogen/ethaneselectivities remained constant at approximately 20 and 44,respectively, across the range of pressures. The hydrogen/propaneselectivity decreased from 140 at 35 psia to 66 at 120 psia. Thus, as inthe previous example, the membranes retained useful hydrogen/hydrocarbonselectivity, even at close to hydrocarbon saturation. Thehydrogen/n-butane selectivity was 373.

[0282] As shown in FIG. 7, the nitrogen/methane and nitrogen/ethaneselectivities remained nearly constant at approximately 1.7 and 3.8,respectively, across the range of pressures. The nitrogen/propaneselectivity was 5.9 at 115 psia.

Example 13 Teflon® AF2400 Pure-Gas Permeation Properties—Not inAccordance with the Invention

[0283] Teflon® AF2400 membranes were prepared as in Example 7, exceptusing a poly(etherimide) support layer. The resulting membranes weretested as in Example 7 with pure hydrogen, nitrogen, methane, ethane,propane, and n-butane at 35° C. at pressures ranging from 17 to 165psia. The n-butane was tested only up to 31 psia, 31 psia being about65% of the saturation vapor pressure of n-butane at 35° C. The measuredpressure-normalized gas fluxes are shown graphically in FIG. 8. Thecalculated nitrogen/hydrocarbon and hydrogen/hydrocarbon selectivitiesare shown graphically in FIGS. 9 and 10, respectively.

[0284] As can be seen in FIG. 8, the hydrogen, nitrogen, methane, andethane fluxes remained nearly constant across the range of pressures.The propane flux increased nearly five-fold from 268 GPU at 35 psia to1,310 GPU at 120 psia, and the n-butane flux increased from 400 GPU at17 psia to 1,110 GPU at 31 psia.

[0285] As shown in FIG. 9, the nitrogen/methane and nitrogen/ethaneselectivities were all low and remained constant at approximately 1.1and 1.6, respectively, across the range ofpressures. Thenitrogen/propane selectivity decreased from 3.0 at 35 psiato 1.0 at 95psia, abouthalf the saturation vapor pressure ofpropane at 35° C., thento 0.6 at 120 psia. In otherwords, the membrane selectivity wasinitially low, and the membrane lost its nitrogen/propane selectivitycompletely by about 50% saturation and became hydrocarbon-selective asthe pressure increased towards the propane saturation vapor pressure.Likewise, the nitrogen/n-butane selectivity decreased from 2 at 17 psiato 1 at 27 psia, then to 0.7 at 31 psia, again indicating that themembrane had become hydrocarbon-selective as the pressure increased.

[0286] As shown in FIG. 10, the hydrogen/methane selectivity remainedconstant at approximately 4.4 across the range of pressures. Thehydrogen/ethane selectivity decreased slightly from 6.4 at 65 psia to5.9 at 115 psia. The hydrogen/propane selectivity decreased from 11.6 at35 psia to 2.4 at 120 psia, indicating that the Teflon® AF was beingplasticized by the propane. The selectivity declined to about 5, lessthan half its original value, at a pressure of about 75 psia, which isonly about 40% of the 180 psia saturation vapor pressure of propane at35° C. Likewise, the hydrogen/n-butane selectivity decreased from 7.8 at17 psia to 2.8 at 31 psia, again indicating that the material hadplasticized and lost its hydrogen-selective capability in the presenceof C₃₊ hydrocarbons.

Examples 14-15 Hyflon® AD60 Multicomponent Mixed-Gas PermeationProperties as a Function of Pressure Example 14

[0287] Hyflon® AD60 membranes were prepared as in Example 11 above andwere tested with a gas mixture containing approximately 63% carbondioxide, 27% methane, and 10% propane at 22° C. at feed pressuresranging from 115 to 415 psia. The saturation vapor pressure of the gasmixture is about 915 psia; thus, at 415 psia, the mixture was about 45%saturated. The measured pressure-normalized gas fluxes are showngraphically in FIG. 11. The calculated carbon dioxide/hydrocarbonselectivities are shown graphically in FIG. 12.

[0288] As can be seen in FIG. 11, the fluxes all increased across therange of pressures. The carbon dioxide flux increased from 46.5 GPU to136 GPU. The methane flux increased from 3.1 GPU to 11.6 GPU. Thepropane flux increased from 0.3 GPU to 2.0 GPU. As shown in FIG. 12, thecarbon dioxide/methane selectivity decreased only slightly from 15-to 12across the range of pressures. The carbon dioxide/propane selectivitydecreased from 152 to 68.

Example 15

[0289] Hyflon® AD60 membranes were prepared as in Example 11 above andwere tested with a gas mixture containing approximately 42% hydrogen,20% methane, 25% ethane, 11% propane, and 1.4% n-butane at 25° C. atfeed pressures ranging from 115 to 415 psia. The saturation vaporpressure of the gas mixture was about 1,130 psia; thus, at 415 psia, themixture was about 37% saturated. The measured pressure-normalized gasfluxes are shown graphically in FIG. 13. The calculatedhydrogen/hydrocarbon selectivities are shown graphically in FIG. 14.

[0290] As can be seen in FIG. 13, the fluxes of hydrogen, methane,ethane, and propane increased slightly across the range of pressures.The n-butane flux decreased slightly from 0.23 GPU at 115 psia to 0.20GPU at 415 psia. As shown in FIG. 14, the hydrogen/methane,hydrogen/ethane, and hydrogen/propane selectivities decreased slightlyacross the range of pressures. The hydrogen/n-butane selectivityappeared to increase from 280 to 328 as the feed pressure increased, butthis apparent increase is within the range of experimental error.

Examples 16-18 Comparison of Methane/n-Butane Permeation Properties withHyflon® AD and Teflon® AF2400 Membranes Example 16 Methane/n-ButanePermeation Properties with Hyflon® AD60 Membranes

[0291] Hyflon® AD60 membranes were prepared and membrane stamps weresubjected to permeation experiments using the same general procedure asin Example 1. The temperature was 21° C., the pressure was 115 psia, andthe feed gas mixture contained n-butane in varying concentrations from2-8% and the balance methane. The saturation vapor pressure of n-butaneat 21° C. is about 31 psia; thus, at the highest n-butane concentration(8%), the gas mixture was about 25% saturated. The pressure-normalizedfluxes of methane and n-butane were measured, and the methane/n-butaneselectivities at the varying n-butane concentrations were calculated.The results are shown in Table 12. TABLE 12 n-C₄H₁₀ Mixed-Gas Pressure-CH₄/n-C₄H₁₀ Concentration Normalized Flux (GPU) Selectivity (%) CH₄n-C₄H₁₀ (−) 2 11.4 2.6 4.4 4 11.0 2.4 4.5 6 10.6 2.5 4.2 8 10.5 2.6 4.1

[0292]FIGS. 15 and 16 are graphs showing the measuredpressure-normalized fluxes and the calculated selectivities,respectively. As can be seen, the fluxes and selectivities remain nearlyconstant over the range of n-butane concentrations.

Example 17 Methane/n-Butane Permeation Properties with Hyflon® AD80Membranes

[0293] Hyflon® AD80 membranes were prepared and membrane stamps weresubjected to permeation experiments using the same general procedure asin Example 1. The temperature was 21° C., the pressure was 115 psia, andthe feed gas mixture contained n-butane in varying concentrations from2-8% and the balance methane. Again, at the highest n-butaneconcentration (8%), the gas mixture was about 25% saturated. Thepressure-normalized fluxes of methane and n-butane were measured, andthe methane/n-butane selectivities at the varying n-butaneconcentrations were calculated. The results are shown in Table 13. TABLE13 n-C₄H₁₀ Mixed-Gas Pressure- CH₄/n-C₄H₁₀ Concentration Normalized Flux(GPU) Selectivity (%) CH₄ n-C₄H₁₀ (−) 2 31 4.8 6.4 4 32 4.7 6.7 6 29 4.76.3 8 31 4.7 6.6

[0294]FIGS. 17 and 18 are graphs showing the measuredpressure-normalized fluxes and the calculated selectivities,respectively. As can be seen, the fluxes and selectivities remain nearlyconstant over the range of n-butane concentrations.

Example 18 Methane/n-Butane Permeation Properties with Teflon® AF2400Membranes—Not in Accordance with the Invention

[0295] Teflon® AF2400 membranes were prepared and membrane stamps weresubjected to permeation experiments using the same general procedure asin Example 7. The temperature was 21° C., the pressure was 115 psia, andthe feed gas mixture contained n-butane in varying concentrations from2-8% and the balance methane. Again, at the highest n-butaneconcentration (8%), the gas mixture was about 25% saturated. Thepressure-normalized fluxes of methane and n-butane were measured, andthe methane/n-butane selectivities at the varying n-butaneconcentrations were calculated. The results are shown in Table 14. TABLE14 n-C₄H₁₀ Mixed-Gas Pressure- CH₄/n-C₄H₁₀ Concentration Normalized Flux(GPU) Selectivity (%) CH₄ n-C₄H₁₀ (−) 2 103 71 1.4 4 122 82 1.5 6 92 801.2 8 112 103 1.1

[0296]FIGS. 19 and 20 are graphs showing the measuredpressure-normalized fluxes and the calculated selectivities,respectively. As can be seen in FIG. 20, the membranes are onlymarginally selective for methane over n-butane, and decreasingly so athigher n-butane concentrations.

Examples 19-21 Mixed-Gas Permeation Properties in Modules Example 19Hyflon® AD60 Membrane Module Permeation Properties at 20° C.

[0297] Hyflon® AD60 membranes were prepared as in Example 11. Theresulting membranes were rolled into a spiral-wound module, which wastested in a module test apparatus at 20° C. at varying pressures. Thefeed gas mixture was 65% methane, 10% ethane, 5% propane, and 20% carbondioxide. The saturation vapor pressure of this gas mixture wascalculated to be approximately 1,150 psia. The pressure-normalized gasfluxes were measured and the selectivities calculated. The results areshown in Table 15. TABLE 15 Pressure-Normalized Flux CO₂/CH₄ CO₂/C₃H₈CH₄/C₃H₈ Pressure (GPU) Selectivity Selectivity Selectivity (psia) CH₄C₂H₆ C₃H₈ CO₂ (−) (−) (−) 213 8.1 3.4 1.6 135 16.6 84.5 5.1 315 7.9 3.41.7 117 14.8 69.0 4.6 414 9.2 4.1 2.0 123 13.4 61.6 4.6 515 11.1 4.9 2.3132 11.8 57.3 4.8 615 14.4 6.5 2.6 148 10.2 56.7 5.5 715 16.0 7.4 3.0146 9.1 48.8 4.6 815 18.8 8.9 3.5 148 7.9 42.4 5.4 915 22.8 11.5 4.4 1526.7 34.5 5.2 1,015 29.1 15.8 7.0 146 5.0 20.8 4.1

[0298] As can be seen, the carbon dioxide flux remained relativelystable across the range of pressures. The methane and propane fluxesincreased 3- to 4-fold with increasing pressure, resulting in the carbondioxide/methane and carbon dioxide/propane selectivities decreasing withincreasing pressure. However, even at 615 psia, at greater than 50%saturation, the membrane maintained a carbon dioxide/methane selectivityof 10.

Example 20 Hyflon® AD60 Membrane Module Permeation Properties at 0° C.

[0299] The experiment of Example 19 was repeated, except at 0° C. atvarying pressures. The feed gas mixture was 65% methane, 10% ethane, 5%propane, and 20% carbon dioxide. At this low temperature, the saturationvapor pressure of the gas mixture was calculated to be approximately 915psia. The pressure-normalized gas fluxes were measured and theselectivities calculated. The results are shown in Table 16. TABLE 16Pressure-Normalized Flux CO₂/CH₄ CO₂/C₃H₈ CH₄/C₃H₈ Pressure (GPU)Selectivity Selectivity Selectivity (psia) CH₄ C₂H₆ C₃H₈ CO₂ (−) (−) (−)213 5.3 2.6 1.7 116 21.6 67.9 3.1 315 5.1 2.5 1.6 95.2 18.8 59.5 3.2 4146.5 3.3 1.8 108 16.7 59.9 3.6 515 7.4 3.7 2.1 120 16.2 57.0 3.5 615 12.56.7 3.2 151 12.0 47.2 3.9 715 17.1 10.0 4.2 170 10.0 40.6 4.1 815 22.513.8 6.9 184 8.1 26.6 3.3 915 45.2 36.6 20.5 222 4.9 10.8 2.2 1,015 54.543.7 23.6 224 4.1 9.5 2.3

[0300] As can be seen, the carbon dioxide flux nearly doubled across therange of pressures. The methane and propane fluxes increased 10- to14-fold with increasing pressure, resulting in the carbondioxide/methane and carbon dioxide/propane selectivities againdecreasing with increasing pressure. However, even at 715 psia, atnearly 80% saturation, the membrane maintained a carbon dioxide/methaneselectivity of 10.

Example 21 Effect of Temperature and Hydrocarbon Saturation onSelectivity

[0301] Based on the data from Examples 19 and 20, the carbondioxide/methane selectivity was calculated as a function of temperatureand percent saturation, expressed as the ratio of pressure to saturatedvapor pressure or critical pressure. The results are shown in FIG. 21.As can be seen, selectivity declines with increasing saturation, butremains acceptable even at high saturation levels.

Examples 22-25 Effect of Carbon Dioxide on Plasticization of Hyflon®Membranes Example 22 Hyflon® AD60 Membrane Permeation Properties at 20°C. at Varying Pressures

[0302] A Hyflon® AD60 membrane was made and a membrane stamp was testedas in Example 11 at 20° C. at varying pressures. The feed gas contained30% methane and 70% carbon dioxide. The pressure-normalized gas fluxeswere measured and the selectivities calculated. The results are shown inFIGS. 22 and 23, respectively. As can be seen in FIG. 22, the carbondioxide flux increased only slightly from 63 GPU at 115 psia to 76 GPUat 415 psia. FIG. 23 shows that, as a result, the carbon dioxide/methaneselectivity decreased only slightly from 15 at 115 psia to 12 at 415psia.

Example 23 Hyflon® AD60 Membrane Permeation Properties at −20° C. atVarying Pressures

[0303] The experiment of Example 22 was repeated, except at −20° C. atvarying pressures. At −20° C., the saturation vapor pressure of carbondioxide is about 285 psia. The gas fluxes were measured and theselectivities calculated. The results are shown in FIGS. 24 and 25,respectively. As can be seen in FIG. 24, the carbon dioxide fluxincreased only slightly from 94 GPU at 115 psia to 113 GPU at 215 psia.The flux then increased to 280 GPU at 315 psia, and then sharply to1,430 GPU at 415 psia, indicating that the membrane had plasticizedunder the extreme conditions of low temperature and high pressure. FIG.25 shows that, as a result, the carbon dioxide/methane selectivitydecreased from 36 at 115 psia to 9 at 415 psia.

Example 24 Reversal of Plasticization in Hyflon® AD60 Membrane Module

[0304] The membrane stamps used in the experiments of Examples 22 and 23had been tested for their pure-gas permeation properties before theywere used under the high-pressure, low-temperature conditions thatcaused them to become severely plasticized. After the plasticizationexperiments had been completed, the membranes were retested with thesame set of pure gases. The results of the tests are shown in Table 17.TABLE 17 Pressure-Normalized Flux (GPU) Selectivity (−) Before TestAfter Test Before Test After Test O₂ N₂ CO₂ CH₄ O₂ N₂ CO₂ CH₄ O₂/N₂CO₂/CH₄ O₂/N₂ CO₂/CH₄ 55.0 17.2 135 4.1 47.8 14.5 137 6.5 3.2 19.0 3.321.0

[0305] As can be seen, the pre- and post-plasticization-test permeationproperties are essentially the same, within the limits of experimentalerror. The Hyflon® membranes were able to regain their originalpermeation properties. Thus, the plasticization did not causeirreversible damage.

Example 25 Selectivity at Varying Saturation Levels and PartialPressures

[0306] Based on the data of Examples 22 and 23, the carbondioxide/methane selectivity was calculated as a function of percentsaturation, expressed as the ratio of pressure to saturation vaporpressure. The results are shown graphically in FIG. 26. As can be seen,at 20° C., the selectivity decreased slightly, from 15 to 12, over thesaturation range. At −20° C., the selectivity decreased sharply from 36at about 30% saturation to 9 as the gas mixture approached saturation.

Example 26 Mixed-Gas Nitrogen/Propylene Permeation Properties withHyflon® and Cytop® Membranes

[0307] Hyflon® and Cytop® membranes were prepared and membrane stampswere subjected to permeation experiments using the same generalprocedure as in Example 1. The temperature was 23° C., the pressure was165 psia, and the feed gas mixture contained 90% nitrogen and 10%propylene. The saturation vapor pressure of propylene at 23° C. is about160 psia, so the gas mixture was only about 10% saturated. Thepressure-normalized fluxes of nitrogen and propylene were measured, andthe nitrogen/propylene selectivities were calculated. The results areshown in Table 18. TABLE 18 Mixed-Gas Pressure-Normalized Flux (GPU)N₂/C₃H₆ Membrane N₂ C₃H₆ Selectivity (−) Hyflon ® 50 4.5 11 AD60Hyflon ® 167 17.8 9.4 AD80 Cytop ® 30 2.3 13

Example 27 Comparative Example of Mixed-Gas Nitrogen/PropylenePermeation Properties with Teflon® AF 2400 Membranes—Not in Accordancewith the Invention

[0308] Teflon® AF 2400 membranes were prepared and subjected topermeation experiments using the same general procedure as in Example 7.The temperature was 22° C., the pressure was 165 psia, and the feed gasmixture was 90% nitrogen and 10% propylene. Again, the saturation vaporpressure of propylene at 22° C. is about 160 psia, so the gas mixturewas only about 10% saturated. The pressure-normalized fluxes of nitrogenand propylene were measured, and the nitrogen/propylene selectivity wascalculated. The results are shown in Table 19. TABLE 19 Mixed-GasPressure-Normalized Flux Selectivity (GPU) (−) N₂ C₃H₆ N₂/C₃H₆ 151 1760.85

[0309] As can be seen by comparing Examples 26 and 27, the membranes ofthe invention provided exceptionally high nitrogen/propyleneselectivities that ranged from about 9 to 13. In contrast, the Teflon®AF 2400 membranes were essentially unselective, but slightly favoredpermeation of ethylene over nitrogen.

Examples 28-31 Comparison of Olefin/Paraffin Separationusing Hyflon®AD60 Membranes (According to the Invention) and Polyimide Membranes (Notin Accordance with the Invention) Example 28 Olefin/Paraffin SeparationProperties with a Hyflon® AD60 Membrane Module

[0310] The spiral-wound Hyflon® AD60 module prepared in Example 19 abovewas tested with a gas mixture comprising approximately 60% propylene and40% propane at 30° C. at pressures ranging from 65 to 165 psia. Thesaturation vapor pressure of the gas mixture at 30° C. is about 177psia; thus, at the highest pressure tested, the gas mixture was nearsaturation. The measured pressure-normalized propylene fluxes are showngraphically in FIG. 27. The calculated propylene/propane selectivitiesare shown graphically in FIG. 28.

[0311] As can be seen in FIG. 27, the propylene flux increased fromabout 6 GPU at 65 psia, to about 9 GPU at 115 psia, and to about 19 GPUat 165 psia. As shown in FIG. 28, the propylene/propane selectivitiesremained essentially constant in the range 3.0 to 3.3 across the rangeof pressures.

[0312] Another spiral-wound module prepared as in Example 19 wassubjected to the same tests and yielded very similar results. Thepropylene and propane fluxes are shown in FIG. 29 as a function percentsaturation, expressed as the ratio of pressure to saturation vaporpressure. As can be seen, the fluxes increased only gradually, andremained essentially stable up to about 80% saturation.

Example 29 Olefin/Paraffin Separation Properties with a PolyimideMembrane Module

[0313] Asymmetric, microporous poly(vinylidene fluoride) [PVDF] supportmembranes were prepared. Composite membranes were prepared using acoating solution of 1 wt % of the polyimidepoly(3,4,3′,4′-biphenyltetracarboxylicdianhydride-2,4,6-m-phenylenediamine) [BPDA-TMPD] in ahexafluoropropanol/chloroform solvent.

[0314] The support membranes were dip-coated inthe BPDA-TMPD solution at0.5 ft/min coating speed, then dried in an oven at 100° C. for 30minutes. The resulting membranes has a selective layer approximately 0.2μm thick. The membranes were rolled into a spiral-wound module, whichwas tested in a module test apparatus with pure oxygen and nitrogen todetermine the integrity of the membrane. The module was then subjectedto experiments at 16° C. and 60° C. at 75-95 psia. The feed gas mixturewas approximately 60% propylene and 40% propane. The saturation vaporpressure of the gas mixture at 16° C. is about 125 psia, and at 60° C.is about 350 psia; thus, at the highest pressure tested, 95 psia, thegas mixture was about 76% saturated at 16° C., and about 27% saturatedat 60° C. The pressure-normalized gas fluxes were measured. The resultsare presented in FIGS. 30 and 31, graphs of the propylene and propanefluxes, respectively, as a function of feed pressure at the twotemperatures. For both gases at the lower temperature, the fluxesincreased sharply above 85 psia, that is, above about 68% saturation.Because the increases in the gas fluxes were proportionate, thecalculated selectivities remained between about 2.5 to 4 across thepressure range.

Example 30 Recovery of Separation Properties in a Polyimide MembraneModule

[0315] The BPDA-TMPD membrane module used in Example 29 was operatedcontinuously for eight days in the module test apparatus with a 60%propylene/40% propane gas mixture at 65 psia and 30° C. At the end ofthis time, the module was retested with pure oxygen and nitrogen. ABPDA-TMPD membrane stamp, which was also subjected to the tests ofExample 29, was tested with pure oxygen and nitrogen for comparison. Thegas fluxes were measured and the selectivities calculated. The resultsare summarized in Table 20. TABLE 20 Pressure-Normalized Flux (GPU)Selectivity (−) Config- Before Test After Test Before Test After Testuration O₂ N₂ O₂ N₂ O₂/N₂ O₂/N₂ Stamp 22.9 4.3 5.5 2.0 5.3 2.8 Module6.0 1.1 4.7 2.3 5.6 2.0

[0316] The stamp and module oxygen/nitrogen selectivities werecomparable prior to the tests. After the tests, the oxygen/nitrogenselectivities decreased significantly in both configurations. Theresults indicate that the damage done to the polyimide membranes as aresult of long-term exposure to the hydrocarbons was irreversible.

Examples 31-33 Comparison of Nitrogen/VOC Permeation Properties usingHyflon® AD60 Membranes (According to the Invention) and PolyimideMembranes (Not in Accordance with the Invention) Example 3lNitrogen/Dimethylethylamine Mixed-Gas Separation Properties

[0317] Hyflon® AD60 membranes were prepared as in Example 11, andmembrane stamps were subjected to permeation experiments using the samegeneral procedure as in Example 1. The temperature was 21° C., thepressure was 65 psia, and the feed gas mixture containeddimethylethylamine (DMEA) in varying concentrations from 3.2-16.6%(16.6% is saturation) and the balance nitrogen. The pressure-normalizedfluxes of DMEA and nitrogen were measured, and the nitrogen/DMEAselectivities at the varying DMEA concentrations were calculated. Theresults are shown in Table 21. TABLE 21 DMEA Mixed-Gas Pressure- N₂/DMEAConcentration Normalized Flux (GPU) Selectivity (%) N₂ DMEA (−) 3.2 10.40.06 163 7.5 9.5 0.08 115 13.5 9.1 0.13 73 16.6 8.8 0.15 60

[0318]FIGS. 32 and 33 are graphs showing the measuredpressure-normalized fluxes and the calculated selectivities,respectively. As can be seen in FIG. 32, the nitrogen flux remainednearly constant over the range of DMEA concentrations; the DMEA fluxincreased as the DMEA concentration increased. As a result, thenitrogen/DMEA selectivity decreased as the DMEA concentration increased,as shown in FIG. 33. The membranes retained acceptable flux andselectivity even in the presence of DMEA at saturation.

Example 32 Nitrogen/Triethylamine Mixed-Gas Separation Properties

[0319] Hyflon® AD60 membranes were prepared as in Example 11, andmembrane stamps were subjected to permeation experiments using the samegeneral procedure as in Example 1. The temperature was 21° C., thepressure was 65 psia, and the feed gas mixture contained triethylamine(TEA) in varying concentrations from 0.7-1.9% (1.9% is saturation) andthe balance nitrogen. The pressure-normalized fluxes of TEA and nitrogenwere measured, and the nitrogen/TEA selectivities at the varying TEAconcentrations were calculated. The results are shown in Table 22. TABLE22 TEA Mixed-Gas Pressure- N₂/TEA Concentration Normalized Flux (GPU)Selectivity (%) N₂ TEA (−) 0.7 17.5 0.08 220 1.6 16.6 0.11 151 1.7 16.20.28 58 1.9 15.8 0.25 63

[0320]FIGS. 34 and 35 are graphs showing the measuredpressure-normalized fluxes and the calculated selectivities,respectively. As can be seen in FIG. 34, the nitrogen flux remainednearly constant over the range of TEA concentrations; the TEA fluxincreased as the TEA concentration increased. As a result, thenitrogen/TEA selectivity decreased as the TEA concentration increased,as shown in FIG. 35. Again, the membranes retained acceptable flux andselectivity even in the presence of TEA at saturation.

Example 33 Comparative Example with Polyimide Membrane

[0321] A polyimide membrane (BPDA-TMPD) was prepared as in Example 29,and membrane stamps were subjected to permeation experiments using thesame general procedure as in Example 1. The temperature was 22° C., thepressure was 65 psia, and the feed gas mixture contained 1.6%triethylamine (TEA) and 98.4% nitrogen. The pressure-normalized fluxesof TEA and nitrogen were measured, and the nitrogen/TEA selectivity wascalculated. The results are shown in Table 23. TABLE 23 Mixed-GasPressure-Normalized Flux (GPU) N₂/TEA Selectivity N₂ TEA (−) 6.2 6100.01

[0322] As can be seen, the polyimide membrane is clearly TEA-selective,in contrast to the membranes of the invention, which maintain usefulnitrogen/TEA selectivities throughout the range of TEA concentrations.

Examples 34-36 Process Designs Example 34Hydrogen Recovery

[0323] A computer calculation was performed with a modeling program,ChemCad V (ChemStations, Inc., Houston, Tex.), to illustrate the processof the invention as reflected in the recovery of hydrogen from refineryoff-gas destined for the fuel header. The process was assumed to becarried out as shown in FIG. 36. Referring to this figure, refineryoff-gas stream 201 at 200 psia passes to compressor 202 where it iscompressed to 400 psia, stream 203. After passing through the compressoraftercooler, 204, the gas is passed as feed stream 205 to membraneseparation unit 206. The membrane separation unit was assumed to containmembranes, 207, providing gas fluxes consistent with the membranestaught in the detailed description of the invention, for example,Hyflon® AD60.

[0324] The flow rate of the raw off-gas was assumed to be 5 MMscfd, andthe gas was assumed to contain 35% hydrogen, 5% nitrogen and 60% C₁-C₆hydrocarbons, of which 15% were assumed to be C₃₊ hydrocarbons. The rawgas was assumed to be at 200 psia and 33° C. The permeate side of themembrane was assumed to be at 20 psia. The results of the calculationsare summarized in Table 24. TABLE 24 Stream 201 205 208 212 213 Flow(MMscfd) 5 5 1.4 1.4 3.6 Pressure (psia) 200 400 20 200 400 Temperature(° C.) 33 60 60 40 63 Dewpoint (° C.) 33 49 −76 −49 58 Component (vol%): Hydrogen 35 35 90.0 90.0 13.6 Methane 30 30 6.5 6.5 39.1 Ethane 1515 0.4 0.4 20.7 Propane 10 10 0.2 0.2 13.8 n-Butane 3 3 <0.1 <0.1 4.2n-Hexane 2 2 <0.1 <0.1 2.8 Nitrogen 5 5 2.9 2.9 5.8

[0325] The hydrogen-rich permeate stream, 208, is withdrawn from themembrane unit and passes to compressor 209, where it is recompressed to200 psia, stream 210. After passing through the compressor aftercooler,211, the hydrogen product stream emerges as stream 212 for use as ahydrogen source in the refinery. Obviously, if the hydrogen were notneeded at pressure, the second compressor could be omitted. The residuestream, 213, now at close to its dewpoint, is withdrawn from the feedside of the membrane unit. This stream is reduced in volume from 5MMscfd to 3.6 MMscfd and in hydrogen content from 35% to 14%, and wouldbe suitable for sending to the fuel header. The process of the inventionrecovers about 70% of the hydrogen originally in the raw off-gas inreusable form.

Example 35 Carbon Dioxide Removal/Hydrocarbon Recovery

[0326] A computer calculation was performed with a modeling program,ChemCad V (ChemStations, Inc., Houston, Tex.), to illustrate the processof the invention for the recovery of carbon dioxide, pipeline gas, andnatural gas liquids from associated gas produced by oilfield floodoperations. It was assumed that the process involved treatment of theraw associated gas by Hyflon® AD60 membranes, followed by treatment ofthe remaining hydrocarbon-rich gas by membranes selective for theheavier hydrocarbons over methane. In this way, the process was able todeliver three product streams: a carbon dioxide stream suitable forreinj ection into the formation; a natural gas liquids (NGL) stream; anda light methane-rich stream, containing only 4% carbon dioxide, suitablefor acceptance into a natural gas pipeline. The flow rate of the rawassociated gas was assumed to be 20 MMscfd, and the gas was assumed tobe of the following composition: Carbon Dioxide 60.0%  Methane 23.5% Ethane 7.0% Propane 6.0% n-Butane 3.0% n-Pentane 0.5%

[0327] The process was assumed to be carried out as shown in FIG. 37.Referring to this figure, gas stream 301 at 415 psia is passed as thefeed stream to the first membrane separation unit, 319, which wasassumed to contain membranes as in Example 34. Carbon dioxide permeatesthe membrane preferentially to produce permeate stream 303, whichcontains almost 97% carbon dioxide and is suitable for reinjection. As aresult of removal of carbon dioxide, the first residue stream, 302, isenriched in hydrocarbons, thereby taking the hydrocarbon content beyondthe dewpoint and creating a two-phase mixture. Stream 302 is mixed withthe light-hydrocarbon-enriched off-gas, stream 315, from separator 325.The mixed stream, 304, is passed to the first phase separator, 320, fromwhich is withdrawn a small liquid hydrocarbon stream, 306. The separatoroverhead stream, 305, is passed to the second membrane separation unit,321, which was assumed to contain the same membranes as in membrane unit319. The second residue stream, 307, is passed to the second phaseseparator, 322, from which is withdrawn an additional liquid hydrocarbonstream, 310. The second permeate stream, 308, is mixed with firstpermeate stream, 303, to form carbon dioxide-enriched stream 309 forreinjection. The second separator overhead stream, 311, is passed to thethird membrane separation unit, 323, which was assumed to containsilicone rubber membranes. Methane-enriched residue stream 312 may bepassed to the pipeline directly or after additional treatment. Permeatestream 313 is mixed with overhead stream 317 and recompressed incompressor 324. This stream is mixed with C₂₊-hydrocarbon-enrichedstreams 306 and 310, and passed as stream 314 to the third separator,325. The separator overhead stream, 315, is recirculated to the firstresidue stream for additional hydrocarbon recovery. The C₂₊-hydrocarbon-enriched bottoms stream, 316, is lowered in pressurethrough valve 327 and passed to the fourth separator, 326, from which iswithdrawn a natural gas liquids product stream, 318. The separatoroverhead stream, 317, is mixed with the third permeate stream foradditional hydrocarbon recovery.

[0328] The results of the calculations are shown in Table 25. TABLE 25Stream 301 302 303 304 305 306 307 308 309 Gas Flow 20 15.2 4.8 20.620.6 — 12.3 8.3 13.1 (MMscfd) Liquid Flow — — — — — 17.5 — — — (bpsd)Flow (lbmol/h) 2,196 1,668 528 2,262 2,259 3.0 1,352 907 1,435 Pressure(psia) 415 415 20 415 415 415 415 20 20 Temperature (° F.) 68 58 63 5959 59 66 62 63 Component (mol %): Carbon Dioxide 60.0 48.4 96.7 39.539.5 17.3 8.2 86.2 90.0 Methane 23.5 30.1 2.6 32.2 32.2 6.4 47.1 9.9 7.2Ethane 7.0 9.1 0.5 14.4 14.4 12.6 22.2 2.7 1.9 Propane 6.0 7.8 0.2 9.89.8 25.4 15.7 0.9 0.7 n-Butane 3.0 3.9 0.1 3.7 3.7 28.0 6.0 0.2 0.2n-Pentane 0.5 0.7 - - - 0.5 0.5 10.3 0.9 - - - - - - Stream 310 311 312313 314 315 316 317 318 Gas Flow — 11.3 5.4 5.9 — 5.4 — 1.1 — (MMscfd)Liquid Flow 667 — — — 1,634 — 1,687 — 1,070 (bpsd) Flow (lbmol/h) 1121,241 591 650 884 594 289 119 170 Pressure (psia) 415 415 415 20 415 415415 20 20 Temperature (° F.) 66 66 22 44 66 63 63 −62 −62 Component (mol%): Carbon Dioxide 3.4 8.6 4.0 12.8 11.6 14.4 6.0 13.0 1.1 Methane 9.950.5 69.7 33.0 28.0 37.9 7.8 18.6 0.3 Ethane 18.8 22.5 17.9 26.5 27.829.2 25.0 43.3 12.2 Propane 34.0 14.1 7.0 20.5 22.5 15.3 37.3 22.8 47.5n-Butane 27.7 4.0 1.3 6.5 8.6 3.0 20.2 2.2 32.9 n-Pentane 6.3 0.4 0.10.7 1.3 0.2 3.6 0.1 6.0

[0329] The 20 MMscfd of raw associated gas entering the system yields13.1 MMscfd of carbon dioxide for reinjection, and 5.4 MMscfd ofpipeline-quality natural gas. In addition, nearly 1,100 barrels per day(bpsd) of natural gas liquids (stream 318) are recovered.

Example 36 Olefin/Paraffin Separation

[0330] A computer calculation was performed with a modeling program,ChemCad V (ChemStations, Inc., Houston, Tex.), to illustrate the processof the invention for the separation of olefin/paraffin mixtures as mightbe necessary in a petrochemical manufacturing plant. The stream to betreated was assumed to contain 80% propylene and 20% propane. Themembrane separation process was assumed to be carried out in a singlestage as shown in FIG. 1. The membrane, 3, was assumed to be as inExample 34. Stream 1 is the feedstream, stream 4 is thepropylene-depleted residue, and stream 5 is the propylene-enrichedpermeate, which may be recompressed if necessary and recycled to themanufacturing process. The feed gas was assumed to be at 150 psia and25° C. The results of the calculations are summarized in Table 26. TABLE26 1 4 5 Stream Mass Flow (lb/h) 3,724 540 3,184 Pressure (psia) 150 15015 Temperature (° C.) 25 24 25 Component (lb/h) Propane 773 307 467Propylene 2,951 234 2,717 Component (mol %): Propane 20.0 55.6 14.1Propylene 80.0 44.4 85.9

We claim:
 1. A process for treating a gas mixture comprising a desiredgas and an organic vapor, the process comprising the following steps:(a) bringing the gas mixture into contact with the feed side of aseparation membrane having a feed side and a permeate side, theseparation membrane having a selective layer comprising: a polymercomprising repeating units having a fluorinated cyclic structure of anat least 5-member ring, the polymer having a fractional free volume nogreater than about 0.3; (b) providing a driving force for transmembranepermeation; (c) withdrawing from the permeate side a permeate streamenriched in the desired gas compared to the gas mixture; (d) withdrawingfrom the feed side a residue stream depleted in the desired gas comparedto the gas mixture; (e) passing at least a portion of the permeatestream as a feed stream to an adsorption unit adapted to preferentiallysorb the organic vapor; (f) withdrawing from the adsorption unit anon-adsorbed product stream enriched in the desired gas compared to thegas mixture.
 2. The process of claim 1, wherein the desired gas isselected from the group consisting of hydrogen, nitrogen, oxygen,methane and carbon dioxide.
 3. The process of claim 1, wherein theorganic vapor is selected from the group consisting of methane,ethylene, ethane and C₃₊ hydrocarbons.
 4. The process of claim 1,wherein the polymer is formed from a monomer selected from the groupconsisting of fluorinated dioxoles, fluorinated dioxolanes, fluorinatedcyclically polymerizable alkyl ethers and perfluorinated polyimides. 5.The process of claim 1, wherein the gas mixture is selected from thegroup consisting of a natural gas stream, an associated gas stream, arefinery gas stream, a petrochemical plant gas stream, and an airstream.
 6. The process of claim 1, wherein the desired gas is hydrogenand the organic vapor is a C₃₊ hydrocarbon.
 7. The process of claim 1,wherein the permeate stream is compressed prior to step (e).
 8. Aprocess for treating a gas mixture comprising a desired gas and anorganic vapor, the process comprising the following steps: (a) bringingthe gas mixture into contact with the feed side of a separation membranehaving a feed side and a permeate side, the separation membrane having aselective layer comprising a polymer having: (i) a ratio of fluorine tocarbon atoms in the polymer greater than 1:1; (ii) a fractional freevolume no greater than about 0.3; and (iii) a glass transitiontemperature of at least about 100° C.; and the separation membrane beingcharacterized by a post-exposure selectivity for the desired gas overthe organic vapor, after exposure of the separation membrane to liquidtoluene and subsequent drying, that is at least about 65% of apre-exposure selectivity for the desired gas over the organic vapor, asmeasured pre- and post-exposure with a test gas mixture of the samecomposition and under like conditions; (b) providing a driving force fortransmembrane permeation; (c) withdrawing from the permeate side apermeate stream enriched in the desired gas compared to the gas mixture;(d) withdrawing from the feed side a residue stream depleted in thedesired gas compared to the gas mixture; (e) passing at least a portionof the permeate stream as a feed stream to an adsorption unit adapted topreferentially sorb the organic vapor; (f) withdrawing from theadsorption unit a non-adsorbed product stream enriched in the desiredgas compared to the gas mixture.
 9. The process of claim 8, wherein thedesired gas is selected from the group consisting of hydrogen, nitrogen,oxygen, methane and carbon dioxide.
 10. The process of claim 8, whereinthe organic vapor is selected from the group consisting of methane,ethylene, ethane and C₃₊ hydrocarbons.
 11. The process of claim 8,wherein the polymer is formed from a monomer selected from the groupconsisting of fluorinated dioxoles, fluorinated dioxolanes, fluorinatedcyclically polymerizable alkyl ethers and perfluorinated polyimides. 12.The process of claim 8, wherein the gas mixture is selected from thegroup consisting of a natural gas stream, an associated gas stream, arefinery gas stream, a petrochemical plant gas stream, and an airstream.
 13. The process of claim 8, wherein the desired gas is hydrogenand the organic vapor is a C₃₊ hydrocarbon.
 14. The process of claim 8,wherein the permeate stream is compressed prior to step (e).
 15. Aprocess for treating a gas mixture comprising a desired gas and anorganic vapor, the process comprising the following steps: (a) passing agas mixture comprising a desired gas and an organic vapor into anadsorption unit adapted to preferentially sorb the organic vapor; (b)withdrawing from the adsorption unit a non-adsorbed product streamenriched in the desired gas compared to the gas mixture; (c) withdrawingfrom the adsorption unit a tail gas stream depleted in the desired gascompared to the gas mixture; (d) bringing at least a portion of the tailgas stream into contact with the feed side of a separation membranehaving a feed side and a permeate side, the membrane having a selectivelayer comprising: a polymer comprising repeating units having afluorinated cyclic structure of an at least 5-member ring, the polymerhaving a fractional free volume no greater than about 0.3; (e) providinga driving force for transmembrane permeation; (f) withdrawing from thepermeate side a permeate stream enriched in the desired gas compared tothe tail gas stream; (g) withdrawing from the feed side a residue streamdepleted in the desired gas compared to the tail gas stream.
 16. Theprocess of claim 15, wherein the desired gas is selected from the groupconsisting of hydrogen, nitrogen, oxygen, methane and carbon dioxide.17. The process of claim 15, wherein the organic vapor is selected fromthe group consisting of methane, ethylene, ethane and C₃₊ hydrocarbons.18. The process of claim 15, wherein the polymer is formed from amonomer selected from the group consisting of fluorinated dioxoles,fluorinated dioxolanes, fluorinated cyclically polymerizable alkylethers and perfluorinated polyimides.
 19. The process of claim 15,wherein the gas mixture is selected from the group consisting of anatural gas stream, an associated gas stream, a refinery gas stream, apetrochemical plant gas stream, and an air stream.
 20. The process ofclaim 15, wherein the desired gas is hydrogen and the organic vapor is aC₃₊ hydrocarbon.
 21. The process of claim 15, wherein the tail gasstream is compressed prior to step (d).
 22. A process for treating a gasmixture comprising a desired gas and an organic vapor, the processcomprising the following steps: (a) passing a gas mixture comprising adesired gas and an organic vapor into an adsorption unit adapted topreferentially sorb the organic vapor; (b) withdrawing from theadsorption unit a non-adsorbed product stream enriched in the desiredgas compared to the gas mixture; (c) withdrawing from the adsorptionunit a tail gas stream depleted in the desired gas compared to the gasmixture; (d) bringing at least a portion of the tail gas stream intocontact with the feed side of a separation membrane having a feed sideand a permeate side, the membrane having a selective layer comprising apolymer having: (i) a ratio of fluorine to carbon atoms in the polymergreater than 1:1; (ii) a fractional free volume no greater than about0.3; and (iii) a glass transition temperature of at least about 100° C.;and the separation membrane being characterized by a post-exposureselectivity for the desired gas over the organic vapor, after exposureof the separation membrane to liquid toluene and subsequent drying, thatis at least about 65% of a pre-exposure selectivity for the desired gasover the organic vapor, as measured pre- and post-exposure with a testgas mixture of the same composition and under like conditions; (e)providing a driving force for transmembrane permeation; (f) withdrawingfrom the permeate side a permeate stream enriched in the desired gascompared to the tail gas stream; (g) withdrawing from the feed side aresidue stream depleted in the desired gas compared to the tail gasstream.
 23. The process of claim 22, wherein the desired gas is selectedfrom the group consisting of hydrogen, nitrogen, oxygen, methane andcarbon dioxide.
 24. The process of claim 22, wherein the organic vaporis selected from the group consisting of methane, ethylene, ethane andC₃₊ hydrocarbons.
 25. The process of claim 22, wherein the polymer isformed from a monomer selected from the group consisting of fluorinateddioxoles, fluorinated dioxolanes, fluorinated cyclically polymerizablealkyl ethers and perfluorinated polyimides.
 26. The process of claim 22,wherein the gas mixture is selected from the group consisting of anatural gas stream, an associated gas stream, a refinery gas stream,apetrochemical plant gas stream, and an air stream.
 27. The process ofclaim 22, wherein the desired gas is hydrogen and the organic vapor is aC₃₊ hydrocarbon.
 28. The process of claim 22, wherein the tail gasstream is compressed prior to step (d).